Millions of people round the world are still using XP, including many businesses.
Microsoft were hardly going to leave them high and dry by not providing fixes, particularly if it was an important security fix.
Also note that while "Windows" may seem like one product it is in fact made up of dozens and dozens of smaller programs all working with each other and "talking" to each other.
It may be that this latest XP fix was to one of those smaller programs or a fix to an important part of the internals of the product.
When it was announced fixes were going to stop for XP many hackers probably felt that XP was a good target for viruses and malware so have tried to target XP over newer versions of Windows like Windows 7 or 8.
So Microsoft probably felt it was important to issue important security fixes to XP.
Of course it could also have been that Microsoft saying they were not going to support XP any more was a bit of "scare mongering" on Microsoft's part to get people to buy new computers or newer versions of windows.