Ofc, but some ideas are balancable others not. I mean the lack of knights was balanced with eagles that dont feel oppressive at any time. The Khmer building bonus ofc bends the rules of the game but by itself it never was problematic. A bonus introduced that bends the rule of the game and is problematic and leads to subsequent removal of techs and making farming rates worse than generic all together make a bonus really breaking the game because it leads to the devs designing the whole game around it other then designing the bonus it fits into the game.Literally every expansion added some new stuff that "broke" the game. Some exception to a previously thought rule written in stone. Even in the base game itself there were already special cases and exceptions to rules otherwise applying to every other civ.
This entire game is made of exceptions. Khmer so called portal gun farms are no exception (pardon the pun).
A few examples:
AoK: all UU's are created from the castle --> also AoK: goths can create UU from barracks (after UT)
AoK: kts and xbow are units shared among all civs --> AoC: meso dont have kts but have eagles, spanish dont have xbow
AoK+AoC: blacksmith upgrades are for military --> AoFE: inca vills benifit from blacksmith upgrades
AoK to AoAK: you need certain buildings to age up --> AoRR: khmer dont
What you perceive as a something breaking the game, is just that, your perception. It doesnt actually break the game.
Mind you, I do think it needs to be properly balanced, but the mechanic itself is perfectly acceptable.