You are probably correct, but I imagine the SQLite team's dedication to backwards compatibility has things the way they are so that existing systems can user later versions a swap without worrying about changing the SQL using it.

If they’re opt-in, how could the new defaults be a problem for backwards compatibility?

The entire point of "SQLite should have editions" is so that projects can opt into a set of modern defaults for 2026 and not get all of those backwards compatible decisions from 20 years ago.