I doubt if ANY serious damage actually occurred by running it WOT from the start. I would never do that, but then again, others think I'm weird because I care for my equipment like it's all made of gold.
Why not start it up and let it idle for a few seconds? It's just as easy, if not easier, to do that, rather than full throttle instantaneously. I've been around m/c's all my life. Not once do I start them with the throttle wide open.
And, it's a matter of respect to the customer. As a dealer, you want to try your best to please and impress the customer, so they will return in the future. That means doing everything in such a way as not to offend.
In all my years of dealing with dealers of varied merchandise, I've yet to meet one that actually knows everything, vs. meeting lots of um that actually think they do.