I did not know that either. Is it? Did I say anything of that sort?

I thought that the Jolla account only had the username and password. It can be after all shared among multiple devices.

As explained here, the sole purpose of the account is that the Jolla app store keeps track of the subset of apps that you have installed to speed up the working out what needs to be updated. The alternative would be that the phone would download the entire collection manifest each time which would take much longer (and is the reason why it takes so long on the N900). There are other options but, as mentioned in the same post, Jolla did not think about them.