Not sure what exactly you mean by "transponder coverage", but if you mean primary or secondary surveillance radar, this is not true for most of the Atlantic or Pacific.
Radar (both primary and secondary, i.e. transponders) as well as ADS-B require line-of-sight, and oceanic air traffic control has to make due without that, which means operating either via ADS-C (i.e. planes self-reporting their position, autonomously determined via GPS or inertial navigation, via satellite communications) or effectively dead reckoning, augmented by occasional position reports via HF radio, which is called procedural control:
Yes, but note how this press release is mostly explaining what this technology could eventually be used for. I'm not aware of any ACCs actually using it for oceanic control today or having made an announcement of planning to do so in the future.
It's a very exciting technology, sure, but I think it's many years of testing and flawless operation as a secondary data source away from replacing ADS-C on oceanic routes.
Radar (both primary and secondary, i.e. transponders) as well as ADS-B require line-of-sight, and oceanic air traffic control has to make due without that, which means operating either via ADS-C (i.e. planes self-reporting their position, autonomously determined via GPS or inertial navigation, via satellite communications) or effectively dead reckoning, augmented by occasional position reports via HF radio, which is called procedural control:
https://en.wikipedia.org/wiki/Procedural_control