The first paragraph implies that Apple management woke up one morning and decided that iOS would need to be refreshed. It then scrambled to find people and none were available. This is obviously not so, OS development is an ongoing project, so why they cannot have manned up (in more ways than one) over the last N years is a bit of a mystery.
True, which is why this sounds like it's more about unexpected work:
The news article claims that "Apple is struggling to implement the changes on time", which would indicate that there's more that has to be done in the allotted time, than they originally thought.
Perhaps they started the changes, then realized that they had to go back and revamp more stock apps than they had expected, so they temporarily shifted other developers over to iOS to help out.
Sure wouldn't be the first time someone miscalculated the impact of a software change!
(A related possibility, and pretty common to the way Jobs used to do things, is that perhaps Jon Ive has made some last minute tweaks that require extra rewrites.)