As far as I can tell, it's the latter option. Rumble has a whole section of their website explaining how to use OBS with it [0]. The complaints seem to be entirely based around Rumble not being available in a drop-down menu [1]. It's kind of pathetic honestly.
[0] https://studio.rumble.com/help/guides/obs
[1] https://x.com/dhh/status/2108603279345070205
(Sorry for the X link, but that is the only place that I can actually find people complaining about this).
I may be completely stupid but these instructions by Rumble either completely unhelpful or something on my end isn't loading content in that site. But if that's the actual instructions to get OBS to stream to Rumble, I get why just upstreaming it would've been user-friendly.
But apparently I'm a fascist for this type of wrongthink.
What's confusing about using RTMP or a virtual camera? It's extremely straightforward. I genuinely do not understand how you could possibly need more instructions than this. Do you want screenshots or something?
Regardless, this just means that Rumble needs better documentation. It's clearly possible and fairly easy.