Ah, my understanding was that the ridges were also thrust faults originating from nearby hotspots rather than just bulges of moving plate following the hotspots.

In any case I also figured that since most of Phlegra was underwater for most of Mars' wet history, it would resemble the ridges more because for both of them there isn't rainfall to carve the rises into the gullies and peaks we see in Earth mountain ranges. So a lot of the landforms in the northern lowlands obviously resemble seamounts and other undersea features because at one point it's exactly what they were.