Right. So when he zooms out he pans down and says that the bit he's standing on is "8-10 feet down" to the beach.
From there I reckon it's couple of feet from the top of the beach to the sea
And he says his his tripod is 4 feet.
He claims he's "right on the beach", but he just isn't.
In the curve calculator he puts a height of 10 feet. Why? He's already said he's on a platform 8-10 feet above the top of the beach and that his tripod is 4 feet.
His viewer height has to be at least 14 feet.
He says he's 21 miles from the building. I've had a look and that pretty much checks out.
Using 14 feet as the viewer height and 21 miles you get a hidden amount of 180ft, without refraction.
He says the building is 65m. I wasn't able to check that, but he then claims that's 195ft - the amount he got on his curve calculator when he put a viewer height of 10 feet.
So he concludes you shouldn't be able to see any of the building.
But:
a) His viewer height is not 10 feet and
b) 65m is actually 213ft.
So even with his own curve calculator and with no refraction at all you should be able to see the top of the building.
He says you can "pretty much can see all of it.". I found this photo of the building which is from roughly the same angle as he was looking from.
I've done my best to match the sizes of the image I found and a screengrab from his video and:

There is plenty of the building missing.
TL;DR - he's put the wrong value for viewer height in the calculator, he's miscalculated the height of the building in feet, he's not taken account for refraction at all and he's incorrectly claimed that you can "pretty much see all of it" when that just isn't true.