It may be more understandable when you know that two dozen of their employees have left for Apple since 2015 (some highly placed), they're basically saying they believe Apple's internal GPU effort piggyback on Imagination's tech.
Patent violation doesn't have a mens rea component - either you violate the patent by doing exactly what is described in it, or you don't. If you change even one aspect, you're no longer violating the patent. I'm sure the people who wrote the original patented algorithms can easily think of ways to accomplish the same thing that don't violate it.