As long as we're speaking for operator, I'll join in on the fun. I think he's speaking as a bike shop person. They learn to avoid advising mixing and matching. If they do, and a customer has a problem, they usually get the grief. It's fine for a person to experiment on their own if they're willing to deal with the consequences. But many times a customer will come onto a shop to purchase a component for a mix and match project they "heard" works on an internet forum, find out it won't work for them, then want to return the component to the shop, or the customer is PO'd then they have to buy more stuff to make it work. Often, it's easier for a shop to presume everyone's a bonehead, and advise them to take the most fool-proof route. You may start out with good intentions in helping someone with a possible work-around solution, but when it becomes a cluster-f@ck, the customer often holds you responsible for un-f@cking it. Work-around solutions can sometimes be a pleasant surprise, but rarely make a shop money.
Shop employees learn to recognize the signs of headaches before the pain sets in. This can be good and bad. It can help the unknowledgeable or fool hardy avoid hurting themselves, but it can also close your mind to some possibilities.