The point of saying that is that there would not have been any siege at Yorktown without de Grasse and the French fleet.

In a world where they dont make it, are stuck in the Caribbean, recalled to France, etc. Then Cornwallis is able to rest his worn out force from a relatively defensible position on the Virginia Peninsula, and await the arrival of British ships from New York which could either supply and reinforce him, or evacuate his force to New York. The British commander in the Colonies, General Clinton, still in NYC had continued to send smaller forces on separate missions to the South, or as reinforcements to Cornwallis. Including one notable one that raided their way up from Portsmouth commanded in part by Benedict Arnold. And when finally ordered by Clinton to Yorktown, Cornwallis is instructed to construct a port and shore facilities sufficient for large warships or transports. A small French force had already fought a British squadron but been forced to withdraw off the mouth of the Chesapeake in March 1781 losing their first chance to blockade the bay and retired to their North American base of operations in Newport RI.

At this point as Spring turned into Summer 1781, Lafayette, Wayne, and Von Stueben command the Colonial forces in Virginia observing them but not actually placing it under siege, as they lack the numbers, siege train, and ability to trap Cornwallis, as even if pressed he could evacuate across the York River to Gloucester Point. Washington and several thousand French troops under Rochambeau are outside New York City keeping eyes on the main British forces and debating what to do in essence. Do they attack Clinton with this newly energized force and try to take the main British point of resistance in the Colonies, or attempt to link up in Virginia and fight an exposed smaller force with Cornwallis, hopefully before they cna get away. And most importantly what was the destination and intent of Admiral de Grasse who commanded the large French fleet then in the Caribbean along with French troops as potential reinforcements. When Washington received confirmation in August that he would head for Virginia the overall plan for the campaign was basically decided. Washington and Rochambeau would take troops from New York and attempt to march to reinforce the Continentals and Militia in Virginia along with more reinforcements to place Yorktown under siege and hope de Grasse could prevent their leaving by sea and force the issue.

And that is essentially what happened when de Grasse arrived in late August off the Chesapeake Bay to find it unguarded and took up station to await developments. Ironically a force of British ships that had shadowed de Grasse under Admiral Hood had arrived several days earlier, but then sailed for New York to link up with the rest of the British naval forces in the theater. Thus in early September Admiral Graves led a British squadron South which met de Grasse and in some inconclusive fighting was forced to withdraw. A slight tactical victory on day one and a 36 hour standoff was ended when the British withdrew in the face of potential additional French reinforcements meant that Cornwallis was essentially abandoned. The battle was fought on September 5th/6th, the Allied forces were in position to begin the siege on September 28th, and the British officially surrendered on October 19th, the same day a second larger relief expedition was actually launched from New York but too late.

Its possible that Washington might have managed a siege without the French Army, though their many guns, experienced engineers, and physical presence were important, without the fleet though there would have been no cause for a siege to start. The Continental Navy could never have done similar as its operations in the war varied between fleeting shows of force, pinpricks, and bad jokes.