Everything you write would apply for someone on vp level with opportunities and stock option.
Get down from there.
If he didn't directly copy code or steal IP or some amazing trade secrets that he contracted never to share, then there is nothing wrong either legally or ethically.
We're not talking about IP. The company had nothing technical that the writer stole from them.
What he did was not even comparable as a product to what they created!
"the ethics of trying to open source a similar project to your old employers product is morally ambiguous at best" would have applied if he had made a product at all!
I don't think it's bad at all, considering replit is itself built on open-source software, and the CEO is a loud champion of open-source software, and the intern's project only explores one relatively small aspect (number of languages supported) of the problem space while purposefully ignoring all the other stuff necessary to build a competing product