Hmmm ... this is a tough one and one that would likely get two completely different opinions from two different lawyers (I swear they intentionally write ambiguous laws just to keep the litigation machine rolling).
Under the Fair Use clause of copyright law, if the work is not original, then you have no claim to copyright of it. Although another person's website may be "original," the comment submitted by someone else to that website is not the original work of the website or its owner(s). One might argue that it is fair game at that point unless the originator of the comment objects.
Just because a website puts in their terms of agreement that they own the content a user submits, does not necessarily make it so. Under tort law, anything in a contract that is not legal, cannot be legally enforced. In some cases, it nullifies an entire contract.
Honestly, I just don't know.
I would also point out that under Fair Use, the courts have to weigh four factors - one of the heaviest being whether use of the disputed material financially harmed the person claiming to have the copyright. I guess you need to ask yourself if they are losing money by you copying the testimonial. In many cases, they may very well be if a customer buys it from you instead of them, although, then an argument can be made that the testimonial wasn't a consideration in the purchase decision - especially if you have a better price! And, since it existed in both places, it clearly didn't sway them to one website over the other (of course, the other lawyer will then say that if it didn't exist in both places, they may not have bought from you). In other cases - like if the testimonial is republished from the manufacturer's website, that would not be the case, since there is clearly no harm in them buying the manufacturer's product from whomever.
Of course, that's if they legally own the testimonial in the first place, which, as I have already pointed out, they probably don't.
Fun little argument for lawyers to have, isn't it?