I meant that hiding your boredom from everyone on the call is more professional than revealing you are bored, regardless of whether your camera is on or not.

This depends on company culture, but yes increasingly having your camera disabled is considered unprofessional unless everybody else has camera off as well (or if there are a dozen or more people in the meeting. In that case rules change a bit).

We could argue about whether it should be that way (I'm quite unsure tbh), but I would advise people who care about professionalism to default to on.