The post in the OOP was referring to recent US elections, in which Democrats (the party that opposes trump and maga) swept both chambers of congress and broke historical barriers (such as indigenous women in the senate, a black woman as a governor, etc.).

This breaks the republican trifecta, which means starting in 2027 trump is going to have a much harder time implementing his agenda.

Even better, a lot of progressives did well within the Democratic Party, which indicates wider acceptance of progressivism which has been gaining support lately. Progressivism is the closest thing we have in American politics to social democracy, and many in the progressive caucus are also democratic socialists.

This is in contrast with the old-guard establishment democratic party which markets itself as moderate/centrist, but by European standards they would actually be center-right fiscally. Especially since Bill Clinton and the "New Democrats" also known as "Third Way."

So a lot of people are feeling optimistic right now, and AOC compared it to the feeling a lot of millennials had after electing Obama in 2008. Hence "Millennial optimism is back."

It's also in response to Gen Z calling Millennial antics "cringe," which AOC reframed as "earnestness." And so the tweeter says she's blaring Florence and the Machine, because they were popular with millennial hipsters back in the '10s.