What you say is surely not wrong, but there is a bit more to that.
Topics/interests that (when being worked at) have routines and schemes that certain people need to relax, can contribute to it as well. But there can also be a motivation like having a goal in life, a dream one wants to realize and the slow realization of that dream (basically shaping it from the unsculpted mass) then does the rest that you mentioned, feeling happy, getting rewarded for success, trying to succeed even more and more often.
In the case of autistic people, I believe those special interests may contribute to calming down, having just the right effect on the brain to relax and allowing to shut down and disconnect from the environment, but the "temporary" effect might be present nonetheless (the brain changes throughout one's life and might not react to it as before), so what could make those who lost it stick to the "special interest" might be the fixed routine and an avoidance of change (the (former) interest being a big part in one's life and getting rid of it a major change). Though if it is really a dream, a major goal in life, that interest won't be temporary but last until death.
That's my theory/view on this topic.
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Diagnosed with Aspergers.
BSP-errors are awesome.