Vasquez has not yet publicly commented on the movie, but some of his recent tweets, like the following, could possibly be referencing the project:

Might finally finish this first draft tonight, but I'll need YOUR help. A THOUSAND of you must die in the proper sequence. Get it right. — Jhonen Vasquez (@JhonenV) October 26, 2016

UPDATE #2: Nickelodeon officially announced the Invader Zim movie on April 4, 2017.

UPDATE #1: Greenblatt has deleted his post from Tumblr. Here is a screenshot of it:

After we published our story, Jhonen Vasquez denied on Twitter that he is developing an Invader Zim movie. However, as we learned a couple days ago, Nickelodeon forces its creators to go on the Internet and lie to their fans. In other words, we’re sticking to our story.

Here’s Vasquez’s official “denial”: