there does seem to be evidence that maybe a 1/3 or so of people diagnosed with adhd in childhood don't present with any impairment in adulthood (figures are ballpark, but people used to think everyone grew out of it, if only!)

the brain keeps developing till the mid 20s or so (it doesn't stop after that, but it's slower), and adhd is fundamentally a developmental delay (with people showing about a 30% age deficit in terms of their executive functioning, i.e. a 15yr old with EF skills of a 10-11yr old), so it seems plausible that for some they actually do manage to catch up, they were just developing more slowly than their peers, whereas others never fully develop age-appropriate executive functioning skills... it's also possible that early intervention itself (whether through medication or behaviorally) might help 'train' the requisite brain areas and so reduce impairment into adulthood

there's also a secondary component though that you touched on, in that adhd impairment is very context-dependent, and some may find themselves in a position due to their job or other life circumstances where it's no longer impairing, in that case it's not exactly in remission, the biological deficit is presumably still there, but you also wouldn't meet the diagnostic criteria for adhd (as it needs to cause impairment in at least two settings)... basically psychiatry's equivalent of 'if a tree falls when no-ones around does it make a sound' haha

(as someone diagnosed later in life it was kind of the reverse for me, there were hints in childhood but it wasn't too detrimental, however as educational demands increased the impact became more and more severe and I sought help for it, took 5yrs but finally diagnosed at 33!)