It’s not that it can’t do it, it’s that it’s against the terms of service to use it for video.
It for any large files. They want to limit bandwidth usage but not blanket limit everyone. One user using Plex of Jellyfin probably doesn't move the needle.
I'm not saying they can't have that rule, it's their infra - I'm just saying that "a boatload of bandwidth" can be anything, depending on who you ask.
I think it’s more of a “we reserve the right to…” situation than anything else.