It's not a matter of "a bit inconsistent". There is no definition of "long covid". Literally anyone can claim they have it and not be wrong.
> but there’s evidence that both repeated infections do tend to be worse with the omicron variants (possibly due to t-cell exhaustion but I’m not sure if that’s the only factor)
No, there isn't, and "t-cell exhaustion" is not a thing. It's a phrase made up by pop scientists.
> I’ve seen at least one estimate that puts the impact of an asymptomatic Covid infection on the brain as being equivalent to 10 years of normal aging.
There are, sadly, a large number of irresponsible people making hysterical, pseudo-scientific claims with no data to back them up. This is a great example.
Even if it were possible to quantify "10 years of normal aging" on "the brain" (it isn't), the specific claim advanced here -- asymptomatic infection leading to accelerated 'normal aging' -- is not backed by any evidence.