Ha, nope, literally everybody who lifts needs to be as strong as Wendler himself before they can consider training specifically for size.
Because everyone needs to train for strength above all else, all the time and in every situation. There are no comparably-effective alternatives. None. Ever. I think that's what Green is getting at.
I'm pretty sure Wendler can outbench several IFBB Men's Physique competitors. But that doesn't really matter because... goals.
By client, do you really mean yourself? It's 100% okay if you do, because based on your training log last year, I'm guessing that's right around where you are now. Weird coincidence.
In any case, clients pay for results. If you were paid money and the client said, "My goal is to add muscle to my chest and upper body", but you've decided instead to increase his bench press just to hit some mythical strength standard, then you damn well better be delivering progress (measurable size increases) along the way. If you're not, then you're ripping off your client and highlighting your own shortcomings as a trainer.