Anyone can distribute the Spine Runtimes without a Spine license, but they have to attach the Spine Runtimes license, which means the need for a Spine license is pushed down the line to their users. In this way someone without a Spine license can use the Spine Runtimes in game toolkits/etc without having a license themselves. https://esotericsoftware.com/spine-runtimes-license

Presumably, if you’re writing a Spine runtime, it’s because you’d like to integrate it into one of your own projects in an environment that they don’t officially support. I’m not sure why you’d take on that “work” if you didn’t already have a license.

>the person that is making the plugin must also hold a license which is too much, not going to pay to do work.

I suppose the idea is that you're paying to work on something that will make you money. And then if you have a FOSS mentality you can share that for others using your engine.

But I'm sure there's wiggle room if you contact. if you're an up and coming engine and are interested I'm sure Spine only has to gain by having its tech ready to go with it.