The official definition of ‘Full Monzo’, from what I recall Simon saying on the forum in the past, is having your salary or bulk of your income paid in to your Monzo account. It has never been intended to mean you shouldn’t have another account elsewhere.
I was never a fan of the term myself precisely because of the disconnect between the intended meaning and the apparent meaning, as you so amply demonstrate. Not least because for the first few years (particularly when the account was still a pre-paid card, and in the initial changeover to current accounts) the messaging was always emphasising that you could open a Monzo account without switching or closing your old account.
Most people in the UK only have one bank account because banking used to be hard, and because incentives for opening a new account usually require you to switch and close your old account, which gives the impression that you’re only allowed one. With the advent of fintech, I think the percentage of people in the UK with more than one bank account will go up and up.
Indeed, there’s probably a number of people on this forum who have at least four accounts - legacy, Monzo, Starling and N26 - for example.
Finally, it is in the interest of fintech banks to embrace open banking. The easier they can make it for people to try them, the more likely they are to gain new customers.