I’m willing to bet it’s the other way around: most Vivaldi users (or at least the ones that matter in terms of extending Mastodon userbase) don’t have a Mastodon account but have a Vivaldi account already (since they are already Vivaldi users).

I think the way they have done it is the most comfortable for new users. Specially considering that most likely it’ll be Vivaldi users coming to Mastodon, rather than the other way around (since there are better alternatives to Vivaldi for those who value FOSS, which is common in early Mastodon users).

And as mentioned in other comment, you can actually use third party Mastodon accounts, even if the option is not obvious.