Children/adults with autism can be gifted in certain areas - as can children/adults without autism.
A great musician/ artist/ mathematician who is autistic is still a great musician/ artist/mathematician. Being autistic doesn't nullify their talents.
I would take issue with the sentiment in the quote and say that some people that had gifted kids later found that their child was autistic. They are still gifted - it hasn't gone away.
For example there is speculation that Einstein may have been autistic. If he was we wouldn't say "we thought Einstein was gifted then we found out he was autistic".
My eldest was an early reader and I think he could have been diagnosed with Autism at some points of his life due to a range of behaviours including difficulty engaging with peers and a love of lining things up.
He's nearly 16 now and has no diagnosis (we never sought any). He's doing great academically and has friends and a busy life of hobbies.
My son learnt piano from grade 1 to grade 8 in 2 years but the reason I think of this type of learning as gifted/neuro diverse rather than bright is the way he learns. During those 2 years he never played for more than 20 minutes in a day, and instead of counting beats (like I did in my much less productive keyboard learning as a teenager) he would be chatting away about other stuff and asking "what's for tea?".
As someone with an older child my main advice to parents of younger kids in this position would be support the social side - the playdates, the making friends with other parents that facilitates friendships between the kids, if you hear your child talking over their pal gently tell them to listen. The academic stuff will to a great extent take care of itself.
I also try to not emphasise grades. My son will ask me "I got x on my test - aren't you proud of me" and I'll say "I'm extremely proud of you but I don't care what you got in your test if you tried your best. If you tried your best that's all you can do " I tell him often I'm proud of his qualities/behaviour e.g. kind/ well behaved etc. I ask him not to brag about grades at school or things being easy as there will be others that tried twice as hard for less good grades and others that don't find it easy. I'm super aware that there are many very clever kids that can't cope if they don't get top grades and find life very hard so I'm trying to raise him to believe grades aren't the be all and end all.
The main thing I wish I could turn back the clock and do differently is to enjoy him more as a young child. I spent a lot of time worrying - mostly due to the drip drip drip of other people's opinions. Whether our children are Autistic, not Autistic or fall into a grey area they have awesome qualities and we should enjoy them all we can - god know we will worry about their difficulties.
Reading this thread I have been struck by the number of people who lost their child or sibling too early, and also the parents whose children are unwell or struggling. I am truly sorry in each case.