No, I think the king does have cancer. I think the timing and the way our attention has been diverted to it is to try and distract us from a potentially seriously ill Kate?
But the timing isn't in their control - he had to cancel engagements so they had to say something, it's not like he could have carried on as normal without mentioning anything was wrong.
And it doesn't make sense to distract from one ill royal with another, more senior, ill royal.