I heard that the reason why music identifiers don’t work offline is because their databases are huge, with millions of songs (including video game music). Databases containing only video game music may be significantly smaller, even if they contain the soundtracks of over 1000 games.
You had me wondering so I looked it up
I dropped a zero, it’s 500MB (and may be a bit bigger now perhaps)
https://venturebeat.com/media/how-googles-pixel-2-now-playing-song-identification-works