There are a number of moving targets in this question, since coverage is continuously changing and while the paper quoted in the question is from 2022 it used a source from 2019.
Satellite coverage is complicated, for geostationary satellites it is not possible to give reliable coverage at the poles, so Inmarsat has very complete coverage, but misses Antarctica and some ocean.
Systems using multiple lower satellites will oscillate around the equator, limiting coverage to certain latitudes. Irridium seems to have handled that problem with polar orbits and accepting the 'inefficiency' with multiple birds duplicating coverage of high latitudes.
Systems using Tundra orbits give small region coverage with few satellites but no coverage outside those zones.
So it would appear in 2023 there is 100% coverage of earths surface for satellite communications. The quality/cost of the service actually available may be variable though!
A further complicating factor is actual availability of service. Systems that need line of sight to ground stations can only provide voice service (rather than text delivered 'sometime') where they have ground stations, which brings financial and legal complications. So the as of 2023 the Starlink coverage map shows near global satellites but only small areas of active service, and an rather less coverage for Canada/Norway/Russia.
We may also see future governments jamming or just arresting users in their territory, does this impact the 'coverage' map?