The canonical way to say this inside Google would be closer to "I created the Flutter project and launched it externally" instead of "founded"

founder is a word that exists outside the context of SV and has never exclusively been the domain of startups.

Guess you're right. Establishing a "Flutter org" counts. Though, I've never seen folks in this space (Bjarne Stroustrup or Anders Heljsberg for example) say they're the "Founder" of XYZ language / framework.

https://dictionary.cambridge.org/dictionary/english/founder (n)

  someone who establishes an organization