It is unusual for Apple to make such a statement about its future plans since they're talking about a couple of years.
And I don't understand why they didn't buy the company since they have plenty of cash. If they're building a custom GPU from the ground up they're going to need a lot of expertise, since they are already stakeholder of Imagination why don't just buy more stocks and acquire the company with all its patents and employees?
What statement? From the story, it appears Apple advised the company privately giving them a heads-up that their products would not be needed in the future.
Apple likely didn't purchase the company because they felt what the company offered was not in alignment with Apple's plans. Apple has enormous talent internally with respect to graphics processing and chip development.