aren't all european countries cheaper than the USA in terms of salary, including Germany?

They are, but Germany has pesky expensive things like unions, employee contracts, and benefits. Keep heading east and it gets an order of magnitude cheaper than Germany.

I would have expected countries like Poland to have a mature enough IT ecosystem that Google could have just moved there directly if they wanted. Why stop over at one of Europe's most expensive places to get talent if that was to goal?

because it's easier to get Polish, Czech, Ukranian engineers to move to Munich than Dutch, UK, German etc. to move to eastern europe I assume