Two labels aren't mutually exclusive.
Avoidant because PTSD is rooted from feelings and beliefs, purely how one is treated regardless of ability.
Avoidant because of ASD is mostly not being able to perform within one's developmental skill.
One can have PTSD because of someone's lacking due to ASD.
But not the other way around -- as PTSD do not cause ASD, but PTSD can reveal ASD if series of events fits the narrative.
Not all those with ASD will develop PTSD.
Common, sure, but PTSD is still a reaction over adverse experience, social or not.
Attachment issues starts at home, and is not necessarily through PTSD.
Though PTSD can cause attachment issues in earlier or later life, ASD may complicate it's process and be prone to it due to societal cluelessness over it.
I insist that those two things are separate.
One is neurology, the other is a human reaction.
I don't believe PTSD is "a part of autism" like anxiety and depression is assumed to be.
Those are a part of life is sh*t adverse package experiences.
Those are reactions of many people who happened to be autistic in a world that isn't made for them.
And as for one trying to separate the two; that's tricky, especially with alexithymia as a factor in either labels.
I think the main thing that separates them is time and consistency because PTSD is a reaction.
A consequence regardless of one's neurology, not something one always will have.
ASD is not a reaction. Not everyone gets that unlike PTSD.
Having both, however, is serious.