They're a public youtube figure who is physically changing their body and documenting that change for a large audience in real time; how isn't that a social transition? Certainly, they could make some attempt to hide that from friends and family but literally all the evidence is right there, on the largest video streaming site in the world (well, also on their chest).

This isn't a matter of policing feelings, this is a person choosing to take hormones because they want to gain the characteristics of a gender they weren't assigned at birth. In the same way that a person who plays music and is paid for playing that music is a musician, a person who takes hormones and desires to take those hormones to look more or less like one gender or the other is a trans person. And, as u/rougepenguin said, it may well be that they simply have trouble spitting out the words "I'm transgender" and therefore admitting to the discomfort of being the wrong sex.