There's at least one other condition that's commonly misdiagnosed as autism: schizoid personality disorder. I'd imagine at least some who don't quite qualify for an autism diagnosis would be well described as schizoid. There may well be other conditions that also share a lot in common with autism that might be similarly good diagnoses for certain folks who don't quite qualify for an autism spectrum diagnosis. But it's also valid for some folks who don't qualify as on the spectrum to just qualify as not having autism.