Much discussion about the pilot’s responsibilities when receiving a radar service from ATC. But this pilot was chomping along at low level, and even if identified by radar, it is not likely that they could be under radar control. In fact, it’s not clear that they were receiving any service at all, having been refused flight following due to coverage limitations.
If you can see, separation and obstacle clearance remain an obligation on the pilot, even if IFR and under radar control. In this case, this flight was certainly not IFR and the whole point was that he was supposed to stay clear of cloud by own navigation.