You are probably right about that. Now when I think about it, I remember that there was some talk on this forum that they are shutting down. But there was also talk about being able to self host Mutiny, something that is confirmed in that blog post you shared. However, that's not something your typical user will want to do or know how to do.
Still in beta, and they are advertising more as Bitcoin-only exchange service.
They are, yes. But it's non-custodial. As long as you have the keys, you can also view it as a wallet. I see it as Bisq. Bisq is a decentralized exchange but the wallet you create there is non-custodial, with you having its seed phrase.
...but I never heard about Bull Bitcoin wallet, that speaks about it's ''popularity''.
Bull Bitcoin's target audience are Canadians. I have heard it mentioned in that context. For example, Ben from BTC Sessions talks about them in some of his videos.