Comment by teraflop

Not a lawyer, but: Intellectual property rights fall into specific, legally-defined categories, such as copyrights, patents, trademarks, and trade secrets.

I don't see any hints that patents or trademarks are involved in this dispute in any way. Copyright violation seems unlikely, unless the intern improperly kept a copy of source code that he had access to while he was employed.

I could maybe see an argument for misappropriation of trade secrets, but the company would have to be able to make an argument that whatever specific secret information he used gave them a real competitive advantage, and that they took reasonable steps to protect it. (Having talked about their tech stack in engineering blog posts, as the OP alleges, would provide him with a pretty strong defense.)

If there's no IP infringement, then it doesn't matter whether there's a commercial motive or not.