Bigots commonly insist that trans people use "I identify as" rather than "I am" when the transgender people are giving their gender, because the intention is to deny those people the ability to be seen as their preferred gender and instead give the impression that those people are impostors, implying that "identifying" is more akin to "relating" instead of categorization.
Insisting that an ostensibly autistic person use "self-identified" instead of "self-diagnosed" would have the same effect.
If you want to use a proper word that's not "diagnosed", "self-assessed" would be more accurate.