After recreating the moment. I came to the conclusion, that he (by a small pixel) shouldn't have taken damage, which means my old conclusion is wrong. Thank you everyone who had read this post and thank you @NoseQ for clearing things up.

STILL, it is something. Maybe simulating the same moment for yourself will get some answers? Idk man, I just wanted to mention it. Have a nice day everyone.

Well that IS possible and I didn't know that. But it was just something to think about. Thank you.

Not that moment, at 4:36 he was towering with basalt then used magma, opened his inventory to get more blocks and casually continued towering without taking any damage. Again at 4:36! Correction: 4:35 to 4:36

Yes he sneaked at first, then he opened his inventory, which automatically causes him to unsneak. For a tick, he should have taken damage (atleast I think so). Of course I could still be missing something.

There is a random seed 1.16 WR speedrun from TwoLetterName, the time is 12:12 and there is one thing that caught my eye. In his video at 4:36 he is standing on a magma block, but he is taking no damage. While standing on a magma block, you get 1 damage per tick. Is there something I am missing here? (Don't be mad at me if I am missing something, it just randomly caught my eye.) (It also wasn't underwater, he was in the nether.) To make it even clearer, he was towering with basalt then ran out and switched to one magma block, at this moment he had to open his inventory to get more blocks. But while he was in his inventory he was still standing on the magma block. He should have taken one tick of damage, but he didn't. After this he continued towering with another type of basalt.