Honestly, it was the safeguarding lead's decision and you raise a very good point.
If I had to give them an excuse, I'd say with the other content modification and HTTPS inspection we've got a fairly good grasp on YouTube already (as far as that can be said) so I think it might be the fact that the shorts endless-scrolling page adds a bit of anonymity with what content is actually being viewed.
...but I'm making an excuse for them at this point.
I was told to block, went looking for a solution, found one, and shared.