HVDC lines are still relatively rare, most transmission lines do have “grid frequency” as converting to HVDC has significant losses. Of course there is equipment to match frequencies, even for Japan’s 50 vs 60hz system, but again it costs money and lowers efficiency.
Rare enough to make the news should be a freaking hint that something unusual is going on even if you know nothing about he topic. But, in case you where actually confused underwater HVDC cables are far from the norm, which is why people are commenting on it.
As to the topic, if you want to bring up unrelated and experimental storage technology then don’t pretend to be “shocked” that the topic expands.
So even your pedantic objections are misinformed.