Those Mandalorians were the leaders, and yes, that had a ceremonial title of Mandalore. That's not the same as "i fight, so I'm a Mandalorian". They had to prove their worth, and they didn't, and as such, the government therein viewed them as "pretenders" because they wore Mandalorian armor without express rights to do so by being a member of this group.
Boba and Jango were not part of the new Mandalorian empire, which isn't so much an ideal as it was a body of government and a culture. So... I mean, realistically you're just flat wrong and trying to cling to your idea. That's fine, but it doesn't change that you're just incorrect. Boba won't be getting any Mandalorian tag anytime ever.