Do aircraft systems really only use GPS and not the full constellation of navigational satellite systems?
Besides GPS, the GNSS currently includes other satellite navigation systems, such as the Russian GLONASS, and may soon include others such as the European Union’s Galileo and China’s Beidou.
The linked page already says that it reports on the constellation, not just GPS:
"The map uses are color coded overlay to indicate low (green) to high (red) levels of interference with global navigation satellite systems (GNSS). Often just referred to as GPS, there are actually multiple systems beside the US GPS constellation, such as Russia’s GLONASS, Europe’s Galileo, China’s BeiDou, and others."
Like the article states, many people use GPS as a shorthand for GNSS generally. In any case, they're all at similar frequencies, so typically they'll all go out together if there's significant interference.
It doesn't matter too much, aircraft don't rely solely on any GNSS for navigation, because they're all susceptible to similar availability issues. Magnetic, inertial, barometric, and land-based radio systems are also used. One or more of those other systems are used as a fallback when GNSS fails.
Modern phones use all the available navigation constellations, and have done so for years.
But aviation is much more conservative due to its safety-critical nature. Galileo was only just recently (2023) certified for use in aircraft systems by ICAO:
Just for the record, this must have been written ages ago. Today you would rather look up to NavIC joining them as a global system and QZSS operating independently from GPS soon.
Besides GPS, the GNSS currently includes other satellite navigation systems, such as the Russian GLONASS, and may soon include others such as the European Union’s Galileo and China’s Beidou.
https://www.terrisgps.com/gnss-gps-differences-explained/