Well, the one thing I can say with reasonable certainty is that I suspect I know where whoever edited Wikipedia got that information. The official Polish government site for the Operational Mobile Reaction Group (GROM) states that:

The first foreign mission of the GROM soldiers was the participation in the “Uphold Democracy” operation in the Republic of Haiti with the United States Army Special Forces in 1994. The main task of the soldiers was the protection of foreign VIPs in the territory of the Republic of Haiti. This difficult and very responsible task was conducted by Polish soldiers as the only contingent of the Multinational Forces.

However, this doesn't sound quite right, and to explain why I need to go into a bit of detail about how Uphold Democracy took place. As the official US Army history of Uphold Democracy Invasion, Intervention, Intervasion notes, the intended invasion was led solely by US troops.

Now there's a specific reason why I've used intended: the invasion didn't quite go off. Junta leader General Cedras was still tediously negotiating in Port au Prince with nominally independent - President Clinton had approved their effort but had not agreed to be bound by them - representatives Colin Powell, Jimmy Carter, and Sam Nunn on the possibility of finally surrendering authority to deposed President Aristide. However, Cedras had dragged his feet on this for over a year and was generally believed to be acting in bad faith, and the military chain of command had pretty much lost all hope that there would ever be a solution that didn't involve the use of military force.

Thus, in September 1994 the military component of Uphold Democracy - which had been in planning for almost a year, with substantial callups of reservists beginning in late July after Haitian refugees had basically overwhelmed US migrant processing infrastructure that summer and proved to be the last straw - was implemented without consultation of the negotiators in Port au Prince. (The latter were basically informed that the Airborne had already launched and to tell the junta that it was now or never.)

However, something then occurred that to the best of my knowledge has never happened before or since in the long annals of military history: the invasion wasn't just cancelled but was actually called off in the midst of being executed. Cedras was not only shown video of the 82nd launching from Fort Bragg but also had agents of his who'd been watching CNN and various bases call him to tell him that the Americans were coming. This resulted in his complete capitulation while the planes were still in the air and a couple hours away, with the result being the initial invasion wave quite literally turned around in the sky. With relatively little local resistance - which the OPLANs had definitely not been built around, nor had troops received any instructions as to what to do once the initial objectives had been secured - US forces took control of several major cities largely on the fly and without firing a shot. From Intervasion:

As Lieutenant General Shelton [CO of the joint task force] observed in an interview, “Never in my wildest imagination did I think that I would be coming in here with the mission of cooperating and coordinating in an atmosphere of mutual respect.“ The abrupt switch in approach just hours after forcible-entry operations had been put into motion not only necessitated a hasty psychological adjustment but left considerable uncertainty about the situation on the ground in Haiti. Shelton had no clear knowledge that all armed agencies in Haiti would respect the terms of the new agreement. Had a forcible entry been conducted, armed opposition elements presumably would have been destroyed or at least isolated within a few days. Suddenly, according to the new rules and conditions of American entry, Shelton had to transform himself from a soldier into a diplomat. Nothing in JTF 180 planning to that point had prepared him to undertake direct, peaceful negotiations with the Cedras regime, which only hours earlier he had expected to remove by force.

So what resulted from all this was the immediate and unexpected implementation of the second part of the OPLAN for Uphold Democracy, which was to begin restoring legitimate law enforcement authority to the island. That was not ever intended to be the role of the first wave of troops, but rather the second, and had always been planned - unlike the invasion force - to be multinational. In fact, starting in June 1994 there'd been a deliberate attempt to form a substantial Caribbean police force - CARICOM - to replace the very unpopular portion of the Haitian Army that had been performing police duties for the junta, with the specific reservation that "CARICOM would follow the U.S. invasion force into Haiti after the country was secure."

That security took a bit for several reasons, not the least of which that the troops who came in during that first wave were immediately pressed into law enforcement and administration rather than pacification, a mission for which they'd neither really been trained for or even in some cases briefed on before they set foot on the island. Another aspect was that not all Haitian forces were particularly interested in cooperating in the new order, and in one case it took a very nasty firefight between Marines and the Haitian Army in Cape Haitien to get the message across, and yet another was that when given the opportunity liberated Haitians were in general also more than happy to turn the brutality they'd received for years straight back on the police in some rather violent ways. However, by October 1994 an international police monitor (IPM) force had been established that was slightly larger (317 vs 264) than CARICOM.

Based on what GROM did, it sounds very much like their actual role was as part of the international second wave with the rest of the IPM rather than immediately in September, albeit with fairly specific and substantial protection responsibilities. That force transitioned into one under the auspices of the UN and lasted into 1997. Ray Kelly, police commissioner of New York City, ran it; it consisted of not only the Caribbean nations of CARICOM, but also Poland, Israel, Argentina, Bolivia, Jordan, Australia, Belgium, Benin, Netherlands, Philippines, and Danish members, and of the 821 members by that point the United States only contributed 35.

TL;DR: It's most likely that the Polish contingent came to Haiti in October 1994 when the police function started expanding rather than as part of the initial invasion wave that was suddenly and unexpectedly shifted to occupation and law enforcement on the fly. It's still probably technically correct, however, to refer to it as participating in Uphold Democracy though, as the continuing UN police mission didn't begin until several months later.

Edit: general cleanup and clarification, and also tagging /u/MaoGo as they asked an unanswered question that included part of this same subject a few months back.