This is by no means a clear cut situation. The question of whether a piece of code which uses this library is a "derivative work" is not an easy one to answer,

This is by no means a clear cut situation. The question of whether a piece of code which uses this library is a "derivative work" is not an easy one to answer, and the GPL only applies in situations where one is distributing said work. Imagine if the client side libraries for Meteor end up in the Google CDN. Then you're only distributing your own code and linking against a separately distributed GPL library. Beyond that, you don't even care if the library is actually Meteor, you just care that it has the API that you use, meaning it can't really be suggested that you know you're linking against GPL code. What about the server/client split? Could you make an argument that they're implementation agnostic to one-another? So why would you need to distribute your server side code just because you distribute your client code? Simple answer is that you don't under the GPL.