Type inference in compiler is context sensitive?
The context that is needed tells us the types of known variables. Suppose We want to infer the type of

if b or 2 = x then a else b


To make sense of this we need to


Check that the type of b is bool
That the type of x is numeric
That a and b have the same type.


To answer these three questions, we look up information about variables in a typing context, which is a mapping from variables to their types. Furthermore, the result is context-sensitive, as the type of the whole expression is the type of a (and b), which is given by the information in the context.

Now you are also mentioning grammars. These are not really relevant here, because we are talking about a stage of compilation which comes after parsing. I think you just got confused by the word "context" being used outside of the subject of grammars. Or you are reading a very strange book. By the way, there is a reason scientists want other scientists to give precise references to their sources. So instead of saying "I read a compiler book", you should have given the author and the title. Now I am just guessing.