I've actually gone really deep on this and the graph they're shows the mechanism at work. "Purple" strictly doesn't exist, you're right, but also wrong. Violet activates essentially the same receptors, "blue cones" in the retina are mainly only sensitive to blue/violet, but if you look at it, the "red cones" actually have an uptick at the extreme of blue (into violet), so when just blue is activated, we see blue, but when we see red+blue, we see it as violet/purple, because if our eyes were seeing actual violet, that's what would be activated.

Purple as red+blue, doesn't exist, it's literally a hack to trick our brain into thinking it's seeing Violet, when it is not.

EDIT: this is a far better explanation than anything I could come up with, and demonstrates the phenomenon. https://jakubmarian.com/difference-between-violet-and-purple/