I know that was in jest, but interestingly it actually is.
I remember this coming up once on the Monzo Community and there was a whole thread showing the Monzo app with test account balances holding millions or billions of pounds, complete with humorously-named account holders!
The screenshots were posted by someone who worked for Monzo to illustrate the point that Monzo were ready for a balance of any figure.
Personally, given the difficulties people have had getting Faster Payment limits raised with them, I would never have an account balance above their daily limit (so that creates a fairly low ceiling, if that’s how you choose to think of it).
Factor in that they don’t pay interest, at all, and it makes no sense to keep large balances in there ever - so it’s not something I’d ever be testing out even if I was fortunate enough to have the money.