They're still providing these to Windows XP Embedded customers like those running it in ATMs, POS systems and industrial control systems until 2019. It's probably been out since march like the other OSes for those users, so they just released the same patch to everyone else.
I surely won't be doing it myself, but I can imagine some spook making this small personal sacrifice of becoming an employee at some Windows XP shop just to smuggle patches to his mothership for vulnerability analysis.
I hope that the fact this patch was signed in February doesn't imply that it was published in February and available to every semi-competent cyberwarfare unit in the world.
QA maybe? Imagine the shitstorm headlines Microsoft would get if they managed to accidentally brick every Windows XP computer in the field with an automatic security update.
https://twitter.com/mikelabatt/status/863356853576749056