The logic is in the article:

Those extreme rarities would already imply a nigh-impossible grind for completionists who want to earn every item in the game on their own.

This is a multiplayer game and economy, not single player. It'd be stupid if every single player got one, so why compute the expected time for each person to get one?

How about we compute the number of people that would get it? A rough estimate

Monthly Active Users * Average Crates per Week per User * Average weeks per month * percentage chance:

20e6 * 3 * 4.35 * * 0.0016 ~= 420,000 users every month that gain the item. Of course these numbers are estimates, probably the average crates per week is way lower, but you get my drift.