About "everything we improved there or modified, we submitted back as open source", the GPL doesn't let you redistribute modified code and only release the modified code unfortunately. I guess "we will release the app you saw as well" may mean that they'd open source the whole app and thus be compliant.
Also the fact that the company was once also known as "Wixpress" is easy to verify [1], not sure why he would contest that.
What an airhead. It's all about feelings and impressions and mutual admiration! No, it's not about those things. You violated a license, and you got called on it. Fix your shit, apologize, and move on. Don't dump a PR load on us without bothering to learn and understand what you did wrong.
Also the fact that the company was once also known as "Wixpress" is easy to verify [1], not sure why he would contest that.
[1] http://files.shareholder.com/downloads/AMDA-29IDMU/0x0xS1193...