That's not correct; you can say sentences like "The only moving parts in a dishwasher are the two pumps, the sprayer arms, the water inlet valve, and the detergent dispenser"; or "there are only two hard problems in computer science". So "one of the only" sounds just fine to me.
you can say sentences like "The only moving parts in a dishwasher are the two pumps, the sprayer arms, the water inlet valve, and the detergent dispenser"; or "there are only two hard problems in computer science".
In the dishwasher example, "only" refers to "moving parts" which is a collective singular, like how "baseball team" is properly an "it," not a "they."
Same goes for the compsci example. By modifying the plural with adjectives, you narrow its scope.
It's definitely not a collective singular, you say "the moving parts are", not "the moving parts is"; and "there are hard problems", not "there is hard problems".
Either way, the usage in the original comment was exactly the same as the dishwasher example: "the only moving parts are X, Y, and Z" => "X is one of the only moving parts".
"8 is one of the only" is a little off because it doesn't have an object, it relies on that being implicit from the prior sentence. "Only" here is an adjective, not a noun. The usage is a little awkward.
It would be more correct to say "there are only 4 positive integers less than 10. 8 is one of them."