Oh please. It does not. That’s just ridiculous. I played a guy with 6 dark angel rifleman dreadnoughts at Heat 1. He quit on turn 2.
2 damage weapons exist. That fact doesn’t totally invalidate Primaris armies, any more that 3 damage weapons invalidate Custodes, or 1-damage weapons invalidate everyone else. There are weapons that are efficient against Primaris, like there are weapons that are efficient against everything.
Sure, if you met a guy whose whole army was autocannons, then you’d have a problem. Ever seen that? Right.
The reason that units like dark reapers are broken is that they are too efficient against too many things, so you’re incentivised to bring a ton of them. Basically any unit that you can spam to win games without thinking is broken.