Why would they lay off the existing team first, rather than spin up the new team, have the old team train the new team, and give old team members a chance to move countries if they wanted?

i honestly don't know, except perhaps to say "this is what google is now". your "rather than" is definitely how it used to be done in the past.

I can only speculate: some directors/vps got headcount reduction goals (reduce eng cost by XXX with deadline at XXX), and they couldn't meet those goals if the layoff is to be delayed until the new team is fully operational.

This. There was a sudden push to reduce costs and people with decision power tried to make sure their teams won't get affected.

Gateway to cheap developers in Eastern Europe, Poland, Ukraine, etc

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