That appears to be the widely held understanding in this particular case.
I'm not so sure. This isn't a strictly black letter law matter. It probably should be, and I'd prefer that it was, but I see political angles to this.
Right now, it is improbable that Trump's DOJ has any interest in doing Ofcom's bidding in the US for UK "online safety" violations, real or imagined. But a world where the US DOJ might does exist. We're the political vectors aligned differently; say, for example, Ofcom was pursuing 4chan for "supporting" ISIS in the UK, I think few people would be surprised learn that Trump's DOJ was eager to "investigate," and perhaps synthesize some indictable offenses, and perhaps even extradite.
Have we not seen, and are we not seeing now, ample examples of similar abuses of power?
So I see much of the rhetoric, and also this lawyer's flippancy, as naïve. Given the optimal set of office holders and sufficient moral panic over some matter, Ofcom et al. could very well have real leverage in the US.