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As I understand it the flat mount fork has no threads - the bolts are fed through from the front and screw in to threads on a mounting plate, on which the caliper is then bolted.
Or maybe not, I dunno, at least it looks like the caliper is bolted to something rather than directly to the fork.
It's a bit of a mess but then it's better than PM in the context of the not fucking up your fork discussion.
Everyone fucks things up from time to time (except Tester, probably) - it's then a crap shoot on the thing you fuck up. Get lucky and it's rounding off a bolt. Unlucky and you've ruined a £200 postmount fork.
From a consumer's perspective flatmount and IS are both better standards.