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: