So much misunderstanding here, including in the video.
The reason
IS simply that Apple will not service a product that the user has opened themselves. Even if you just opened it to have a look round, it's considered by Apple to be a product that has been serviced by a third party, and so they won't touch it.
It's not that they can't repair it, it is that they won't.
And yes this is ridiculous and in some places illegal, but that is Apple's policy and they were following the policy.
It is nothing to do with not being able to get the parts due to shortages. It is simply that the Apple Store could not order the parts for a product that has been cracked open by a third party - it is ineligible for service.
No doubt Linus will have a new specced out iMac Pro sat on his doorstep tomorrow morning courtesy of Apple PR, but don't expect this policy to change in the future for you or I - Apple have no interest in touching things that someone else has been inside for as long as they can get away with it.
If, for example, they dropped it on the floor and smashed the screen that way - it would then be eligible for service. Maybe he should go back and say he did that...