Well, OK. You can do that too. But AFAIK there is no documentation (for either procedure) so you would be on your own dealing with the break / fix.
I imagine that this is a deliberate design decision - to give a user experience that is more similar to what people are used to on smartphones. I expect also that with even more effort you could reinstate the standard Linux desktop GUI login screen. That’s the real Linux philosophy, right? You’ve got the source. You own the system. Make it the way you want it.
Personally I don’t like it from a security perspective. It gives an attacker a known username to target. (Is that really a problem today for most users? Probably not because too few Librem 5 phones are in circulation, but we can hope that one day …)
In an ideal world, on first boot after install, it would ask for the desired “main” username, create it and configure to use it - so you would never have to rename the default username in order to get your desired username. (This is basically what happens with some desktop Linuxes.)