Classy would have been, “We respect the decision made by Costco workers to form a union, and we look forward to a mutually constructive relationship.”
You are entitled to your opinion, though I would say that there is some hyperbole to your phrase and word choices and given its directed at a woman it may not be having the effect you intended regardless of how you mean it (I am chronically guilty of this so speaking from I hope experience).
Williamson is only being critical of what they are not saying, and not doing, she is not being critical of what they are saying and are doing. Again both can be true.
Costco clearly highly values taking care of employees. They were masterful in accepting responsibility, though it still has to be proven in their long term actions (which I fully expect it will be).
Williamson is also correct that the response did not go far enough to welcome and encourage the union or more people to join the union. That is the criticism, and as far as I can tell only that.
Costco has a very carefully crafted message by saying “has never been the result of any union” its interesting wording because its designed to get people to react by saying well if a union did not get me the great treatment I already have, then how is this new union going to make things even better.
It subtly delegitimatizes the value of the union while not explicitly attacking it and lets Costco claim a victory when the original conflict was of their own creation and they lost not won. That line pivots from look what a union can do, to you dont need one because we always put you first, ignore that we just owned up to not doing that.