I think they don't want you using a boatload of bandwidth, since data proxies through their servers.

Yes and that's what people are criticizing - it's just an arbitrary and thus very bad rule. Completely unrelated to CloudFlare, I streamed a single TV series from a friend's plex account 1-2y ago, that's less traffic than some of my friends use in 2 weeks.

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.

FWIW, even if it’s against the rules I have been using Cloudflare tunnels for my Jellyfin server for months and I haven’t even gotten a nastygram yet.

I think it’s more of a “we reserve the right to…” situation than anything else.