Using dairy definitely seems to boost the creaminess. One of the recipes in that same feature (the only one that used dairy) included evaporated milk and the resulting rice milk which I found interesting. I’m just spitballing but I think the higher fat content would mimic the resulting texture from a higher concentration of nuts (which would also be much more expensive to include).
I’m not vegan or lactose intolerant (to my knowledge) but I am glad Guisados announces there’s dairy in their horchata. It’s so creamy it reminds me of my favorite horchata from @robert’s neck of the woods, La Mission. After the article I wondered if they use dairy but I haven’t had a chance to ask.