Here is the other relevant part of the X class description:
(X class) This is an exhibition only class. No Zone 8 award points or trophies are awarded in this class. This class is for non-Porsches, Porsche replicas, factory race cars and exotics such as the 917, 550 Spyder, 904 and 959 (or other models not listed in the Base Points chart). This is also a class for those who want to have fun running their car but do not want to interfere with other participants' efforts to win series points or awards.
Note 1: Factory race cars that are based on street cars (found in the Base Points list) may run in the appropriate CC class as determined by their level of modification above and beyond that of the production street car.
Note 2: As stated above, for class competition purposes there are no Zone 8 points or trophies for class X. However, if an individual region has other competitions that are not based on the class structure, such as Top Time of Day or Top 10, etc. rules for those competitions are to be determined by the host region. Inclusion or exclusion of class X in these competitions is at the region’s discretion.
So a CupCar may not have been the best example if you claim it is just a modified 991, but a 919 Hybrid LeMans prototype could also run against a Miata or any Porsche with the same BRI factor? Still seems like a ridiculous idea to place the same BRI index on all cars in X and CC16, just slightly higher than CC15, under the pretense of providing an indication of the drivers ability. The rules actually say no trophies for X cars for any trophy based on the class structure, which the BRI is, so it violates that rule except no trophy is given in TT.
That said, I applaud the rules committee and all the hard work they do. This isn't really a big problem in the grand scheme of things, I was just curious how it came about, not trying to create a problem or say this needs to be fixed.