This is the "aha! got you there!" argument I often hear from those who believe that firms couldn't possibly discriminate AND remain profitable. After all, a firm which could save on its labor costs by not discriminating would make higher profits, right?
The shadow side of that argument is naturally the assumption that women are worse workers and deserve lower pay.
A response to all this can be found in this post.
It's worth reading if you often engage with people making the above argument.