I think that statement causes a lot of misunderstanding. I think that for many the realization of the Holy Spirit is in fact a gradual thing. Some of those think that because they didn't completely change in an instant that they actually are not filled with the Holy Spirit and doubt takes control. The affects of the indwelling of the Holy Spirit and the realization of that indwelling is often a gradual thing.
There is a lot of confusion and opinions about what the filling of the Holy Spirit is and what it should look like or manifest as.
There’s not even consistency in terminology.
Some people say you are baptized into the Holy Spirit when you are saved, and filled later.
Others say you are filled initially upon salvation but baptized later.
I think they’re talking past each other, meaning the same thing but not expressing it clearly.