If Google is interested in acquiring the engineering talent of these at will employees, that seems like a strong signal that they're worth a lot more than they think, no?

At the very least, they should negotiate directly with Google to sweeten their deal, preferably with at least one counteroffer for their talent. On the other hand, if Google is making a better offer than all other comers, then they should just take it. If you don't at least attempt to negotiate, I can almost guarantee you're leaving something on the table.