That's what happens when you reply to a 14-years old thread without doing any research.
First, this post links to a a free utility MADE BY FLIGHT1, so if you wanted to complain about it not working well here, it won't be very effective, just as it would be silly to complain on Flight1 website about, for example, an FSDT scenery not working. So, if you have any issues with this utility, please report them to Flight1, which have a support forum as well, and they might help you with it.
Also, fact this post was made in 2008, should make you see things in perspective, because back then there weren't many airplanes specifically made for FSX, most of the were converted from FS9.
As far as I know, FSX changed the way Rain in VC is made, and requires each airplane develop to create its custom rain effect that is specific for that plane only, while in FS9 there was a "global" effect that worked (the same) for every aircraft. However, FSX still supported it, but only for planes originally made for FS9 and used in FSX.
That's likely the only time in which this fix might be useful, but I don't think many are stil using planes for FS9 in 2022, and it's even possible that later FSX versions ( like the Steam version, for example ), made this fix useless or possibly not working at all.