I feel the concept of a purple patch is greatly misunderstood here. A player can have a period of poor form, even for an extended period, for example, Javier Hernandez went 2 years without scoring a single goal in the Mexican league, then came to the Premier League and scored for fun.
In evaluating whether this was a purple patch or not, you have to look at the context. It's not as if he has been playing these 4 years, he's been benched at City despite his form and then moved to Leicester when Vardy was scoring for fun and was never going to displace him. This is the first time, he's played consistently since his first patch of form. The fact is that when Iheanacho has been trusted, he has always scored at this rate.