There’s a bit of truth in both perspectives.
It is Linux, it is open source, so, given enough time and effort and expertise, you can make it do whatever you want.
It may not be as easy as just saying “It’s Linux, therefore it supports different logins”.
My guess is that, at Evergreen shipment, this functionality won’t be available.
However I think we can all see the merits of the use case set out by @StevenR so perhaps it will happen later on.
It’s not just so that the user experience is customized to the settings of the individual user. It could also be about the privacy of your contacts, emails, texts, … To pick up on Kyle Rankin’s recent metaphorical post: if your phone is your digital castle then if you are going to loan the keys to the castle to a family member then you are going to want multiple logins.
On the other hand, maybe Purism wants your family member to buy his or her own Librem 5.