As stated in your parent and elsewhere, Mega rarely responded to DMCA requests.

Sorry to split hairs but responding with a "we took down this content based on your takedown notice" and actually taking down the content are two different things.

I've had companies take down content after a DMCA notice but never inform me they have done so; there is no requirement for them to do so.

I assume you're referring to the latter meaning though--that they left up infringing content after receiving notice. If they did that then they're in deep. ...As long as the US has jurisdiction to prosecute, which they probably do.