It's interest in the sense of "conflict of interest", not in the sense of finding something fascinating.
"Disinterest" doesn't work at all in any sense of the term. In the absence of overt mental illness, the bus-ticket collector must see it as being in his best interest to spend his time collecting bus tickets, or he'd do something else instead. ("You are your calendar.") The search for gratification, however externally meaningless, is certainly a valid expression of self-interest.
Put another way, if you would object if you were forcibly stopped from pursuing a goal, then you cannot be described as "disinterested."
"Intrinsic motivation", perhaps?
Maybe "impersonal"? Still weirdly contradictory but closer to the meaning maybe.
"detached" or "impartial"?
The limitations of English are weird.
[2] I worried a little about using the word "disinterested," since some people mistakenly believe it means not interested. But anyone who expects to be a genius will have to know the meaning of such a basic word, so I figure they may as well start now.
From dictionary.com, the second definition of disinterested is "having no interest in something," but the first is, "not influenced by considerations of personal advantage."
Because you're not pursuing personal advantage, this kind of obsession is incompatible with competition. You're obsessively interested in collecting old bus tickets not because you want to get paid, not because you want to be famous, not because you want to change the world, and not because you want to win.
So it's a bit weird for pg to identify "heuristics you can use to guess whether an obsession might be one that matters". If you care about whether a thing matters, instead about the thing itself, then you're not really disinterested.
As soon as your obsession becomes influenced by thoughts of personal advantage, then it's about garden variety ambition and determination, not the magical property of disinterest that pg describes.
Disinterested obsession may be a powerful source of innovation and progress, but the instant you intentionally try to harness this power in pursuit of progress, you destroy the magic of disinterest.