Urethane sidewalls have been around for a while. K2 used to do partial urethane sidewalls underfoot on some of their park skis and Ride uses it in their snowboard with "slimewalls". It is generally used to get a softer and more flexible sidewall than can be provided by ABS or UHMW. It, "bends before it breaks" essentially, so it may help to minimize sidewall or edge cracking. It is easier to bond than UHMW, which is a selling point for smaller manufacturers. You can also use different color sidewalls more easily with urethane, because you can just dye it before you pour it and don't have to meet companies minimum orders on UHWM or ABS. I would rank the correct hardness urethane sidewall between ABS and UHMW in terms of best sidewall material. ABS is generally more brittle and prone to cracking. Urethane has it's place, it is more prone to edge and base deformations on impact. Which may or may not be better than a cracked sidewall or edge. UHMW is still in my opinion the best material for sidewalls. It offers a more appropriate mix of hardness and toughness.
Here is a video of ride explaining it and comparing it to ABS.