I assume one of four things.
1- It was a mistake/oversight by Brandon.
2- Weird progression occurs when you break a bond then reforge a new one (similar to Shallan) which might result in both being sworn.
3- Now that spren can leave Roshar (it’s mentioned some Windspren are with the Iriali caravan), maybe the armour is formed by whichever are nearby.
4- He may have been close to the 4th ideal so Windspren may have already started following him, and even though he abandoned his Windrunner oaths, his Windspren may not have abandoned him.
None of these are overly fleshed out theories, but best I can speculate.