@feruze-- Yes, he used already existing Japanese plum strains to make the Santa Rosa plum. He actually founded more than one hundred new types of plums and lots of other fruits and berries!
I've made frozen ice from Santa Rose plums before. It was really good. When Santa Rose plums are ripe, they are very soft and mash really easily. That's why they are perfect for making ice cream, frozen ice and smoothies. It's also great for cobbler. All of these come out sweet and a little tangy and I really like it.