What does “be at it” mean? Is it an idiom?
Solution 1:
As Barrie says "to be at it" essentially means "to be doing it". It is an idiom.
The slight differences are that it is a little bit less explicit and I feel it can also be a good expression to say "to be trying/attempting (to do something)".
Solution 2:
You were pretty much right. The sentence could be rewritten as They've been doing it ever since, although be at it is rather informal.