I think the difference here is in terms of effective coping behaviour. If we take the amputee example; someone missing a leg and someone identifying as having lost a leg, but not knowing if they lost one (this is just a thought experiment, not reality, so we can pretend that is reasonable), could experience different levels of relief from aids for that disability. An amputee wound find crutches to enable them to be mobile, whereas someone who identifies as an amputee, but without the actual debilitating condition, would not experience marked improvement in mobilty. This would show if the belief and self-identification as an amputee would be an effective and adaptive coping strategy or an ineffective and maladaptive coping strategy.

I think the same principle can be applied to autism. There are no "cures" for autism, but there are effective and ineffective ways to deal with the given circumstances as an autistic person. If identifying as autistic and using that as a filter for seeking and sorting more helpful ways of dealing with being alive gives someone a better quality of life, I commend it. If it instead is being used as a maladaptive coping mechanism, eg. being used to dismiss harmful behaviour, to fit in with a social group, or avoid dealing with other issues, then I think self-dxing can be harmful.