Firstly, no they are by far not the only tanks that can be one-shotted from the cannon. They are, however, the only tanks that almost have no weakspots other than that area surrounding it(2A7 and 122).
Secondly, a 13 or 5 second video is meaningless. You only test one time, you shot inaccurately on the Abrams, and you only tested 2 tanks in total.
Thirdly, even if this were true, it would be a deserved nerf to Leopard 2A7, given it has already been the most powerful tank in top tier ground for more than 2 years. The meta is stale and I would go as far as to say completely unenjoyable.