It's interesting that in religion back then they took the idea of the gods' existence as an obvious fact. So when they encountered other people with different gods they assumed that they were actually the same gods, just by different names. If they encountered civilized/intelligent people with different mythologies it meant either that those people knew something the Greeks previously didn't or they had just gotten events confused and it would become a matter of debate. It was more like two people with different, but honest, accounts of an event in the remote past trying to reconcile their own limited recollection or view point.

The Greeks and Romans were doing it long before Alexander, as were all the cultures of the Mediterranean and Ancient Near East as far back as history allows. Egyptians were borrowing Levantine gods early in the Bronze Age, and ancient Akkadian adopted the Sumerian pantheon wholesale.

Turns out, culture isn't a singular thing you can just have. People everywhere all the time are picking up bits of things to do, things to say, things to believe in, and borders, no matter how fluid they are, only exist in the aggregate.

It's more cultural colonization than syncretism. For example, America was culturally colonized by forcibly converting its inhabitants to Catholicism. Their temples were destroyed, the stones used to build churches, and while celebrations were still observed, the deities were replaced with Catholic ones. Even the folklore was rewritten with Catholic characters.

It wasn't syncretism, it was systematic eradication. Syncretism is more akin to reconciliation of different views.

Syncretism is mixing religions (as doctrines, beliefs). Assimilating local traditions into one's religious rituals is not that.

That's not the right word here, as Catholic dogmatic teaching does not enter into negotiation with what you might call competing doctrines. It doesn't blend bodies of doctrine like that.

We're talking about adopting cultural practices in a selective and transfigurative way. If a cultural practice doesn't conflict with the Church's teachings and would be beneficial, then it could be embraced as such. A cultural practice may also be transfigured and adapted into something that maintains a certain resemblance, but has a changed and elevated meaning. For instance, having encountered a culture that practices ancestor worship, it might take that practice and transfigure it into a restrained expression of filial piety that moves it away from idolatry. Pagan harvest festivals are another example where the symbolic meaning is changed.