I agree. It would be very hard for a lawyer to prove that using a public API amounts to derived work. Even so, GPL allows modified but not distributed versions (like you would see on a meteor server). The author of this blog post does not seem to fully understand GPL.
You're trying to make a complex issue simple. It's true that scenarios exist where you can use the redistribution-loophole to keep your modifications to a server side GPL program proprietary (Affero GPL was later designed to prevent that).
Meteor's client and server architecture are very tightly intertwined, and both of them are GPL (v2). Given that the GPL is (deliberately) vague in defining the technical details of copyleft, I can't see how it would be unreasonable for an attorney to take the position that the Meteor client and server parts form one program and thus when the client part is redistributed, the server part must be too.
Heck, I would be surprised if folks like the FSF didn't take that position. "The client and server sides raise different ethical issues, even if they are so closely integrated that they arguably form parts of a single program." -- Stallman himself in [The JavaScript Trap](http://www.gnu.org/philosophy/javascript-trap.html).
I agree that the poster only has a superficial understanding of the GPL. Interestingly in the case of Wordpress themes the Software Freedom Law Center has argued that these are derivative works, even though one could argue that these merely use a API cf http://wordpress.org/news/2009/07/themes-are-gpl-too/
