If a builder has finished digging a building but hasn't moved yet, it takes no damage from attacks. We can see this in one of our very own approved runs from user Caylus:

You can see this exploited (even more) for The Shattered Kingdom (TSK) mission 8.

Yes, you may use this glitch, as it was a mistake of the original developers (wasn't intentional, but technically legal). However, of course please do not waste your (or our) time playing a bugged mission like the following (play in a different version of the game which is not bugged)!

Have fun!