You can be sued if engineers decide to quit and come work for you?
So yes. You can be sued for that. Obviously. Which is why he's getting sued.
Whether or not the litigant prevails depends on specific language of agreements, their enforcability, and the specific circumstances around the departures.
Did the engineer decide to quit on their own or were they persuaded by someone who otherwise agreed not to poach?
The issue discussed here is an employee agreement that prohibits former employees from poaching current employees.
I don't see how it doesn't fit this case.