I told somebody I know who knew about Reddit's API changes about Lemmy. He has a master's degree in Computer Science and works as a software engineer. But then, he told me that it's too confusing to get into, even for someone like him. This is great feedback and I hope that these issues will be fixed in the coming months.
It seems to me that he follows the link to another server and is asked to log in again. Different server, another account is needed.
Or that, yes. Technically you don't need another account for another server.
For example, this link is relative to your home instance. But if I just paste the full link: https://discuss.tchncs.de/c/lemmytips it probably shows the page logged out.
Related GitHub issues: #3259, #3261
Technically you need another account if you want to post on another server.
For some reason the user landed on a different server with the same look and feel. When challenged to login (again in their experience) they got confused their credentials wouldn't work. The new user expected a single-sign-on for 'lemmy the social medium'.
Could be an early adopter issue that'll be solved over time with wider use and more content, and people get to know what to expect from federation.