Read it before. I take everything with a grain of salt, especially when it comes from people who sell me things, even if I respect their expertise.
I agree with what they say for the most part, but I also think that some people underestimate the resilience of some organisms (like yeast). I've never had a long lag phase or stressed yeast myself, no matter how I started my yeast. I agree that a person shouldn't start their yeast in distilled water (duh!) since there are no minerals or dissolved oxygen in it, but find me a genuine logical reason why starting yeast directly in an environment full of food is going to harm the yeast.
the yeast cells will absorb some substances that are toxic to them early
what substances? By what the FAQ said, nitrogenous compounds help the yeast early on: The presence of 1/2% yeast extract, yeast hulls, autolyzed yeast or peptone in the rehydration water will give the yeast an added boost that will get it through its lag phase quicker.
So is it sugar? Some mineral or acid?
I mean, a good start is important and I generally always rehydrate my yeast in tap water first, although for the sake of experimentation I've pitched it straight into the fermenter and starters before, and I've had no signs of unhealthy yeast. The lag phase was not unreasonably slower than had I rehydrated the yeast, and there was none of the rhino farting of a stressed yeast going on, nor was there any detectable HS2 in the product. Moreover, I haven't really read anything about yeast metabolism to indicate that pitching dry yeast into a must would harm it in any way, and I like to read a lot of papers and books on microbiology.
Sorry to sound like a dink. I just don't buy it, based on what I've read and what I've observed.