Something similar happened with Half Life 2 - and it went on to be a smash hit.
Yes, it's not nice to have your work revealed before its ready, but locking down and just keeping the pace of development up would be the more judicious use of time.
I'd agree, and also point out that Half Life 2 was far more damaging. This is a bunch of tech development demos and trials, there's little to no story spoilers. Whereas HL2's leak contained levels and story elements that appeared in the final game. It was unfinished/alpha, but really showed what HL2 would eventually be, whereas this just shows some of the technology that GTA VI may contain.
I don't even read too much into the diner scenario, since that's a great test-bed for interactive NPC behavior, object interactions, and general scenario creation tooling. It MIGHT be in the final game, but it may also not be, and it wouldn't be impactful either way.
Yes, it's not nice to have your work revealed before its ready, but locking down and just keeping the pace of development up would be the more judicious use of time.