Yes. You are right. But how does it differ from any other test done by a car magazine? So tell me, which magazines do an in-depth analysis of the car at test, whether it is stock or not? They basically can't! They would have to get test cars anonymously from a dealer - all controlled by a notary. Have you seen that?
So in the end it is all about trust!
The youtuber cited here has some trustability. He is owning/driving many different makes. So he shouldn't be biased to one company. If he wanted to cheat: why should he then tell about which gas he had filled up in which run? Since he is regularly tuning his cars over time: how should he explain upcoming performance jumps? Or in case there are none, because he already cheated in the first place and his 765LT wasn't stock.
And finally: Those times are absolutely plausible, if you know the many existing dragstrip times of various 720s and take into account the optimizations in the 765LT regarding gearing, weight and power ...