Wait — so you took a GPL-ed plugin, created a wrapper around it, and then released that wrapper under the MIT license — which to me sounds fine and reasonable except that it violates the GPL. GPL derivative works need to be GPL-ed. It is then a question of enforceability.
Matt is extending the scope of this GPL derivative work by including your entire app, under the truest of GPL definitions, which makes this a case for the lawyers and a judge to decide based on examining the facts should it go down that rabbit hole. They would win this, in my opinion, but I am just a blogger on the Internet.
GPL has the Midas touch — anything it touches becomes gold.
I am sorry as a developer you are in this situation — you did something clever and good and are unnecessarily stuck up in this GPL-bullshit of a situation. Hope your company stands by you.