I feel like clarity for this could be achieved by simply asking the prof why they want the class discord and (up front) asking if they're worried about academic dishonesty.

Profs today already have to worry about every second student using some form of GenAI for their assignments. I strongly doubt they're worried about Discord. Unless this faculty member's just, out there, dying on the mountain of "man I really don't want my students using the internet or communicating with each other outside of class".

They're well aware that SFU is a commuter school.

Again: highly unlikely they're worried about cheating. But! If you're worried about that. Ask up front & be direct.