The reason segwit effectively increased the block size was because it moved some data outside of the transactions, but didn't change the block size limit. I could also be very wrong.
I think you are right and it sort of compresses the block but not by using Gzip but moving some of the data
to what they call an "extended block" so maybe the files will look more like
BLK000001.dat
BET000001.dat
Here I am guessing but from a coding perspective it will make it more complicated having to read/parse two files
and will break existing code so I suspect I am missing something key here.