This is a well documented and well known failing of the Arduino IDE preprocessor. It doesn't respect (or simply ignores) the preprocessor conditionals, and will aggressively match any line starting with #include for a library inclusion.
It's been reported as a problem for years. Like many other problems with the IDE preprocessor, Team Arduino don't think it's a big deal, apparently, so it's not going to be fixed.
My solution is to simply not use the IDE to avoid the numerous bugs and problems in the IDE preprocessor altogether. I use makefiles that do the build. I need to declare my own prototypes, and put a list of user libraries I'm using in a makefile script for that project. Done. No IDE preprocessor. Real C/C++ preprocessor directives all work as expected again. Life is better that way.