Yes. But when you don't know what it is you may start to believe that it's just you being lazy, stupid, slow, difficult, etc. Especially when everyone around you including teachers use these labels and you literally have no other information to base your views on. A mix of living with that for decades and being unable to know how life feels for other people can make you gaslight yourself very thoroughly.
A diagnosis is an acknowledgment of an authority on the subject that you are in fact not crazy and don't have to blame everything that happened on yourself.
> Leaving the reaction for the diagnosed to be mostly nothing in that they already know
I am glad for everyone who experienced it like that, but that's not a reason to diminish the significant effect for many others.