As for the GDSes, their code is primarily TPF assembly. So now add the LoC blow-up of using a very low-level language.
There are like 25 people in the world who understand this stuff, and half of them likely work at ITA/Google.
As to the "categories", people at Sabre working with them used to say in some cases you can not even be sure if the calculation (of a ticket price - "fare") will end in finite time, so the "macro language" is apparently Turing-complete!
I've written elsewhere that I personally think the original Sabre system was/is one of the most impressive accomplishments in the history of computing. Using modern tools made things easier for us at ITA, though we compensated for that by trying to compute the entire (very large) solution space for every query, where prior systems used heuristics.