Basically the “low-tech” your are seeing is absorbed human technology being used for the purposes of human control. Obviously the advisors that are the primary combine race (i think) arent going to have some kind of futureistic gas mask invented for humans, especially since they seem to want to live in a “polluted” world (look through breens portal at end of half life 2 and they are draining earths resources to adapt it to theyre own likeing kinda like teraforming).
If you look closely you can see the combine does have advanced technology, the striders, the pulse weapon, which by the way has its secondary fire which is equivelent to the half life soldiers useing theyre grenades from the smg so they arent as dumb as you say, the combine gunships are another high tech example, the citadel and its contents(Teleporters, the core, the energy orb tech)
The combines abilitie to turn humans into cyborgs is another technological feat. If you try playing on hard(how its meant to be played) you’ll see that combine armor can take more then 5 bullets. Again, the lowtech is just absorbed human technology. Unless you think cyborgs, advanced robotics, synths, teleporters and a towering mechanical structure is lowtech, in which case, your just retarted.