Considering they have a login system, I'm going to guess that the cookie includes your login (probably in JWT form), which automatically makes it essential to site functionality. Which means the banner is there just because if it was absent, someone would say "Hey, where's the cookie banner?"
In other words, it's not actually legally required in their case, but it's practically required, because it lets everyone know that the absence of the banner is not a violation of the law.
