As someone who was born with unilateral moderate->severe hearing loss, I can read lips. The experience is likely unique across the hearing loss spectrum / time of onset, and some people may be able to learn that skill themselves, idk. I'm sure 100% deaf people experience it in their own interesting way.
To me it's not anything I conciously do, and it's not something that's really that visible to me. The fact I can still hear, but not as well as people with normal hearing affects how it works. The way I'd explain it for me is kinda like this:
Sometimes I can't hear enough to tell what is being said, one way my brain naturally deals with this is by reading the speakers lips and using that to help filter and understand what its hearing. I can kinda apply it as a skill, like with muted videos and people I can't hear because of distance, but it doesn't work that well and isn't worthy of trust.
So for me it's more of a sense, not something I do or think about. However, its basically the least effort way to understand speech that isn't clear enough. This is in contrast to another way I/my brain goes about it, which is trying really hard to figure out what it just heard.
To answer your last question, yes it is likely a mistake. Theres a youtube channel about that whole concept, called bad lip reading or something. They dub over video with audio that matches the lips well enough.
To put my experience into perspective, which might work for at least a few people: closed captions ~~subtitles~~. I mean... I've never asked anyone else but yall arent just reading them, right? To me they just clarify the speech subconsciously (for the most part), rather than me reading them off the screen when I need them. Captions are weird... Who knows if this is accurate to my experience or similar to others.