An API can be used as a GPL propagation route only for as long as there's a single implementation of it that's GPL'd. And only potentially at that. For example

An API can be used as a GPL propagation route only for as long as there's a single implementation of it that's GPL'd. And only potentially at that. For example, if I "publish" an API: int strlength(const char *); and ship a GPL code for it, I cannot realistically expect anyone depending on strlength() to be suddenly liable for GPL'ing their work. That'd be ridiculous. Meaning that for the Blender add-ons to not be subject to Blender's GPL clause it should be sufficient to have an alternative implementation of Blender's API. This, incidentally, will also validate addons against GPL criteria for "derived software" - whether the code in question can or cannot function without GPL-licensed software. If it can, then it's not a derived work and not a subject to GPL terms. The only way for Blender to prevent commercial addons (which is ultimately what their actions amount to) is to explicitly prohibit that in their license.