Of course if it was already assigned when IANA said that (no dates on the link above) then maybe the fault is more IANA's for telling another RIR that they could allocate an ASN that another RIR already allocated.
i suspect that, in the erx project, there may have been more than one case of the iana saying "ok, X now manages this block, excpet of course for those pieces already allocated by Y and Z." and the latter were not always well defined or easily learnable, and were not registered directly with the iana, but other rirs. <rant> and the data are all buried in whois, which is not well-defined, stats files, which are not defined, etc. the rirs, in the thrall of nih (you did know that ripe/ncc invented the bicycle), spent decades not agreeing on common formats, protocols, or code. this is one result thereof. testosterone kills, and the community gets the collateral damage. randy