This is a difficult question to answer, as we know very little about Thracian religion in general, and most of our evidence comes from later centuries. Moreover, the Thracians actually consisted of many different peoples (often described in English as "tribes") across a large geographic area, so the evidence that we do have for Thracian religious practices and beliefs may not be representative.

First, let me address the idea that the "Thracians at the time believed only in a handful of Greek gods, not including Zeus." This notion presumably stems from a claim by the Greek historian Herodotus, one of our earliest and most important literary sources on the Thracians. In his brief overview of the Thracian pantheon, Herodotus explains:

They worship only these gods: Ares, Dionysos, and Artemis. But their kings, in contrast to the rest of their countrymen, worship Hermes above all the gods and swear only by him, saying that they descended from Hermes. (5.7)

If Herodotus' observations here have any basis in reality, he has almost certainly described Thracian deities in terms of their Greek equivalents. Artemis, for instance, might correspond to the Thracian Bendis, with both goddesses being associated with hunting, torches, and dogs. Yet Herodotus contradicts himself as well, for elsewhere he refers to other Thracian deities, such as the enigmatic Pleistoros, to whom the Apsinthian Thracians allegedly sacrificed an unfortunate Persian captive. (9.119). Pleistoros, presumably, had no Greek equivalent in Herodotus' eyes.

Looking beyond Herodotus, we find further signs of religious syncretism and even shared use of sacred spaces in communities where Greeks and Thracians lived side-by-side. Notably, excavations at the temple of Apollo in Zone, a Greek city on the southern coast of Thrace (opposite of the famous island of Samothrace), have turned up over two hundred ostraca from the sixth and early-fifth centuries that feature writing in an indigenous Thracian language (though mostly utilizing Greek letters), the majority of which appear to be dedications to Apollo. Later traditions attest to the local worship of a manifestation of Apollo named Zerynthios, whose origins may be Thracian or Thracian-influenced. In short, both Greeks and Thracians in this region seem to have worshiped a form of Apollo.

As another example, we can look to the city of Odessos on the western Black Sea coast, where archaeologists discovered a small sanctuary built in the late-fourth century that apparently belonged to a local Thracian god named Heros Karabasmos (he's depicted in reliefs as a "Thracian Rider"). Significantly, this sanctuary stood within the precinct of a temple of Apollo (Odessos' chief god)---some later evidence suggests that Heros Karabasmos and Apollo may have eventually been assimilated with each other---illustrating how Greek and Thracian deities could share sacred spaces and, presumably, worshipers.

In sum, if a Greek and Thracian met in 529, they could probably point to various commonalities between their respective pantheons ("You call your hunting goddess Artemis? We call her Bendis.") However, when we examine evidence on the ground in communities with both Greek and Thracian inhabitants, it's often difficult to separate the Greek element from the Thracian; thus, while Thracians in Zone may have worshiped Apollo, their conception of Apollo may have been intertwined with local religious beliefs about Zerynthios.

I unfortunately don't have time to flesh out this response further, as I have to rush out the door now. Happy to provide sources upon request when I get home (most of this comes from my dissertation research).