I guess it's just someone who's rlly popular in the community who says it and a lot of people go along with it. Not trying to be shady but I remember when rookie 1O1 said that Ning Ning had very shallow A4s and G#4s and strained B4s and a lot of people agreed and went along with it.
Yeah and the sub vocal supported range thing isn't as common but I guess it is still kinda annoying. I can get why they think that. Like for example, Chaeyoung sometimes (on rare occasions) sounds more relaxed that usual and people mistake that as shallow support. Same goes for Mina.