There is no "Not spam" button for the inbox, as if something lands in the inbox, Gmail does not consider it to be spam to begin with. There is an actual spam/mark as spam button in the inbox in case they missed something.
In the case of someone who is using the Gmail web interface, the very topic of this message is irrelevant. Gmail does not use the subject line in any way to differentiate between spam/not spam or even classification into labels/folders.
I really don't know what specific software is in use that puts the various bits within square brackets at the start of the subject line, and I'm not talking about the group identifier itself, which is put there by Groups.io. I've just noticed recently that lots of topics are being split on several groups because things like, "[spam]," or, "[Suspected Spam]," or "[External]," are being stuck on the front of the subject lines. And I'm virtually certain that it's not human intervention that's putting those there. A spam and general filtering program I haven't used for at least 15 years now, Popfile, used to have an option that would do this to subject lines, but I don't know of anyone using it anymore and doubt that's what's going on here.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
Any idiot can face a crisis. It's the day-to-day living that wears you out.
~ Anton Chekhov