Plugin development is modding, though not really considered modding by a lot of Minecraft Forge developers. That is fine. I'm very well aware that Spigot development as well as plugin development is probably a joke, and it's probably not impressive unless you're modifying the server(.jar). The deeper you get into plugin development, the more you learn from it -- including being able to do more than you would normally be able to with the Spigot API, using NMS (net.minecraft.server).
Ideologically, most people want to work through an API anyways. When you want to access NMS in its true, "raw" self,