If you look at the connection map, https://www.researchgate.net/figure/Map-of-United-States-sho..., I guess the high capacity DC line that serves vegas could be siphoning some Utah power into socal, but there is much more capacity in the connectivity from Oregon and Washington.

Reading more, there are two high voltage DC lines into SoCal: one from a coal plant in Delta Utah (2.4 Gw, not just from that plant, but let’s say majority coal), the other from a hydro conversion station on the Columbia in Oregon/washington (3.1 Gw). Looking at the map above it looked like the latter one terminated in Las Vegas but I guess it actually goes into SoCal.