The REPO devs are quoted as saying:

And while on this topic, we want to encourage all mod makers to please optimize the amount of server traffic they send to avoid any skyrocketing server costs.

But the article writer turns this into:

The devs are most likely referring to mods that increase REPO's player count above its 6-player default max. Last year, Content Warning faced similar issues, to the point that the devs had to shut down mods that increased the lobby size.

If that was what they meant, that's what they would have said, since what they said doesn't imply that at all. Depending on the API, the mods could be doing any number of things with even a single player that would send network traffic through the roof.

Telling modders this is just about lobby size might diminish the impact of what the devs are requesting. An inexperienced modder who isn't changing lobby size might not realize they're inadvertently being a bad citizen with other stuff they're doing.

As someone else mentioned, they should look into protecting themselves within their API though. Rate limits, etc. Since this whole thing took off on them unexpectedly, they probably weren't prepared, so in the meantime they're requesting the mods be more careful. Ultimately, this won't work though.

Tbh, if it was just a lobby size thing, I would think that would be super easy to defend against server-side...