That wasn’t always the case, and I don’t know if it’s currently the case. At least at one point, they would intentionally lose money by dropping their prices below profitability just to get mom and pop shops to shut down, and then raise prices back up to profitability. Or they’d force suppliers to cut costs only for them to the point where the supplier wasn’t making a profit, but by then they had stopped selling to competitors.
There’s a lot more evidence for Walmart committing anti-trust than Valve.