For the tiny number of people who would use them it’s not worth monzo risking their reputation if one of those sites got a virus laden/fake version. There’s really no case for uploading an app outside the play store where there’s very careful checking both automated and manual.
The amazon app store is the only one with any reputation close to that… and it’s debateable whether it’s worth spending any time uploading to that as so few people use it.
Aptoide clearly doesn’t do any checking as they allowed a third party to upload a copy of the monzo app without permission. That’s basic. If they’re not even doing that then that’s a huge red flag to stay away. It might be unmolested this week (you have no way of verifying that)… but you’re letting this thing do automatic updates - the one tomorrow might empty your account.
On the monzo site is possible but really why? I doubt it’s even 1% of android users without a play store account.