His line here does look very strong but his value range is very limited. What full houses can he realistically have? 88, 66 and 86s. Then there are the higher flushes, but since you're holding 2 of the diamonds the likelihood of being flush-over-flushed is quite small. KK shouldn't be in his range since he limped (Although this theory may be flawed because he still limped with 88) He could also take this line with hands you beat, such as A8, 89, 78 and 8T.
I think this is a call on the river despite his strong line as he can easily take this with worse hands than your flush