We never said PCs are worth skipping over for games (at least I didn’t). And if your older games work on your system you’re lucky, many older games don’t work without using programs like DOSBox or Windows compatibility settings.
Look at steam update logs every once in a while. “HL2: Fixed crashing for ATi 9800 Pro” is an example of things you would find (just made that up, but there are plenty of logs similar to that). So yes, they do extensive testing on a wide variety of hardware combinations before releasing the game, and quite often have to release a series of patches later for those people who own hardware/software that isn’t compatible