Is there a difference between 他这是 and 他就是? A graded reader (for foreigners learning to read Chinese) has this sentence: 他们这是在做坏拿！

I think this means the same as: 他们就是在做坏拿！And I think both are just emphatic ways of saying 他们是在做坏拿. Google Translate treats all these sentences alike.  But I would like to hear from people who know.

Is 他这是 just another way of saying 他就是?
I only use 這是在 when I actually see it. Like "他們這是在做什麼?". But for the similar "他們是在做什麼?" without the 這, you can use it regardless of the place or time of the object.

For 就是在, it just basically means "It IS" or "They ARE", when you emphasize the aux verb.