The het highway, het superstripe, ect. all got named that first before their modern name came around.
As for the terminology, well it doesn't even come close to making sense imo, like even more than usual. when you say het "X" you are referring to an organism being heterozygous at a locus with mutant gene X and its wild type allele both being present. It takes two different alleles of the same gene to be called heterozygous, its not just the mutant gene. Highway is heterozygous at the locus we are referring to, two different mutant alleles. So if anything, the actual highway is the only true het highway. At least in that sense you can breed a het highway to a het highway and get highways
But I think calling them gravels and just moving on is much easier.
You can NOT breed het Highway(Gravel) to het Highway(Gravel) and get a Highway.
Gravel x Gravel gives you a Super Gravel
Gravel x Yellow Belly gives you a Highway.
Super Gravels and Highways look completely different.