This is very important to point out. The manual says 40" or when the head is even with the top of the headrest. I suspect they threw in the headrest thing just to protect themselves from the possibility that a child's head might be taller than that with a standing height of less than 40"
Yes, many people make the decision to use the seat beyond 40", but there's a big difference between using it for a couple more inches and using it to the top of a fully-extended headrest, especially given the potential for extreme downward rotation in a conventionally-installed seat with a very tall/relatively heavy child.
If people want to make a parental decision to ignore the 40" rule, that's their prerogative. However, I feel it's very irresponsible for a tech to state it's absolutely fine, especially without giving any kind of background info or other caveats.