In American English, we say “thumbs-up” even when the gesture is with one hand. So this sentence, in my opinion, doesn’t clearly indicate that she actually used two thumbs. Saying “thumb up”, doesn’t sound correct and isn’t commonly used. Hope this helps!

If you want to indicate how many hands were used to give a thumbs up then you need to say "thumbs up with one hand" or "thumbs up with both hands". You could also say "thumbs up with his left hand". Basically, you need to use "with" to specify with what they did the thumbs up. For example: She gave him a thumbs up with both hands. I gave my sister a thumbs up with one hand while holding my bag in the other. The woman gave him a thumbs up with her left hand because she was holding her baby in her right hand.

No, it is just the way we say it. It just means to put your thumb up. You can put your thumb up on one hand or both. "Give a thumbs up or a thumbs down" is the command for showing like or dislike. You could use one or two hands depending on how much you like it or not. You also might hear, "Give two thumbs up for..." That would mean, put your thumb up on both hands to show you really like something. You could also put both thumbs down if you really didn't like it. I hope this helps.

I think Jessica has answered this perfectly! In South Africa, we often also say "thumbs-up" for a job well done or when something has been achieved. This does not need to come with the action of putting your thumb up, the words can be spoken without any action and the meaning is still understood.