I don't think it's a myth.
People do it subconsciously with more illnesses than ASD/ADHD.
People will try to beat the system. In the 90s I was talking to a physiotherapist who was telling me about a young girl they'd had a few years back, who had injured her knee in a car accident. Every time she tried to get her to do any movement she screamed in pain and flinched away.
At the third session, her dm had to go to the toilet mid-appointment, and the physiotherapist said to the young girl in desperation "how can I help you to try to do the exercises?"
The girl replied totally straight: "I'm not going to do them because my mum says that if I show I can move my knee I won't get as much compensation money."
So people will do their best to use what they can get out of situations; that's not a new thing.
But actually I wasn't talking about that. I was talking more about the power of the mind when people read about conditions, even more so if they think they might have it - and people will research into it.
When dd was about 12yo she came to me and said she thought she had a particular physical ailment. I know a little about this, and I said straight off, that I didn't think she did, why did she think that? She gave me a mild symptom that could show the ailment, but only in conjunction with other much worse symptoms.
About three or four days later she came to me saying that she was sure she had the illness, and listed several of the main symptoms. Obviously I was more worried, and made a doctor's appointment.
That evening her internet report came through and I noticed that she'd been on a blog for this illness. So I went onto them and found her list of ailments were exactly as listed - there were other symptoms which were not listed, which she also hadn't had.
When we got to see the doctor he confirmed absolutely that she didn't have it through a test, he gave medication for the milder symptom and the other symptoms vanished before we left the surgery never to return.
The power of the mind can be very strong, and I think the natural thing for a young person to do if they are wondering if they have ASD or ADHD is to research it, which then lays them open to feeling they have the symptoms even if they hadn't noticed them before - even more so than a physical ailment.