Another example: shooting straight en face with 50/2.8 both eyes are sharp pretty the same, but shot with 300/2.8 = it is easy to say which eye was focused on.
Sure it is, if you are shooting at the same distance - in that case, if the face fills the 300mm frame, it's only a small portion of the 50mm shot, and if it fills the 50mm frame then your 300mm shot is only showing the eyes (or would be, if you weren't closer than the MFD of the lens). But if you're 6 times further away at 300mm compared to 50mm, so the framing is the same, then the DoF is the same
It always has and always will be difficult to convince people that what they think they see is not in fact true (even with explanations and examples).
Through the filter of our own misconceptions, everyone else's statements are part of the blurry background.