If that's all Dr D is arguing, sure he's correct, he's just having an argument by himself. I don't think anyone is arguing it's not a disorder. At least I'm not.
I'm mostly agreeing with your statement that diagnosing a mental disorder in a kid this young is a silly pursuit. I think it's damn near impossible in this case due to the age of the kid and the complexity of this disorder. You can ask a kid that age all the questions you want in whatever fashion you want, but the kid is still limited in how they express themselves verbally. Especially when they can't possibly understand the problem.
Observing symptoms of gender confusion in a kid that age is one thing. Being able to accurately predict if the child actually has the disorder, and whether the disorder will persist into adulthood is another entirely.
But hey, whether the gender confusion is real or not for this 5 year old, and whether it persists or not, as the kid grows up the years of being made fun of for the parents battling the school over the bathroom the kid uses is going to be plenty real.