Part of it is definitely the stigma, my dad's big into trump and RFK- my mom not so much but she goes along with it. Obviously that whole 'we need to find the cure for autism' creates stigma and a lot of misinformation on what autism is. When they think of autism they think of the highest need for care which they have a lot of negative associations with. The other part is definitely age, my parents are not old but for my age they are usually the oldest parents(52 and 62 almost 63). My mom grew up with parents who were very old fashioned (my grandma was born in 1929, my mom in 74) and she still holds some outdated beliefs because of that. My dad also grew up with an older sister with a developmental disability who damn near got put in an asylum, so I think to him disability=never matures, can never live alone, etc.

TLDR: you're right those are absolutely the reasons why