Aha, so I wasn't that far off with my speculation prior to September Patch Tuesday - looking at file dates, the TLS fix had already been completed at that time, but for some reason it was held back for two regular release days. Thanks for keeping tabs on this, @heinoganda!
According to Microsoft, It'll be on WU/WSUS as Optional starting January 16 and will turn Recommended on February 13. (Perhaps they'll fix IE and WinHTTP by the time to make it more useful.)
Just so there's no confusion: Chrome was capable of TLS 1.2 on XP even before this fix. (relevant forum topic)