like the dev said, game optimization isnt as simple as 2d game = run good and 3d game = run bad, theres a ton of shit that goes into it (physics calculations, drawing the sprites itself takes up processing power, calculations for gore, blood, etc.) plus, black mesa runs on the source engine which is... rather old, so i would expect it to run rather well despite the enhancements they made to the engine. and slime rancher runs on unity, not a very intensive engine. those games also have teams and whatnot to help with optimization
mb if this came off as like rude or anything i just dont think the stuff that goes into optimization is really known to the public