You do whatever you want in your videos of course, but that challenge is supposed to be accomplished by fair means. Mods were not mentionned for the simple reason that there were no mods at the time of the challenge. Some fair bp-edit changes or even the use of infinite build area are usually allowed because no kiloton launcher would fit in the build grid anyway (actually it's possible, I made
this one), but stretching engines is generally forbidden, because their thrust scales up with size, but their mass doesn't. And even if that was the case that would just be lame. That's why it's not accepted.
Honestly, using an engine that has an Isp of 3.500.000 seconds like in your Grand Tour video is not very different from using infinite fuel.