I'm also pretty out of date, but I believe in LTE (4G) the max distance was closer to 100km, with the timing advance controlled in the radio protocol. As I understand it, the 5G radio protocol should be fairly similar to 4G, it wasn't completely reinvented for once.
Based on my read of the summary of a couple Mbps per cell and focusing more on low bandwidth services like texting makes me suspect they're right at the limits of what the tech will allow. Since they're advertising compatibility with existing hardware, makes me suspect there is an extension somewhere in the standards that allows the timing window to be increased, or maybe there's some clever hackery going on, like only scheduling every other timing window and cutting bandwidth in half.
Which might work on the uplink, but on the downlink I think it's more complicated to support something like paging, where the UE is mostly idle, and only wakes up occasionally, that things don't get out of sync with the moving satellite.
There's a system using LTE to backhaul Internet traffic from planes (WiFi in cabin, with a LTE to the ground), working with larger cells of up to ~900 km from memory, but it doesn't use standard LTE for example. It uses a modified variant made to support the possibly larger propagation delay coming with larger cells.
For 5G (NR), the max cell size has been increased and depends on the OFDM subcarrier spacing. For a 15 kHz SC, same as LTE, the max cell size is 300 km. This value is halved each time the SC doubles.
There's work on-going to extend NR to non-terrestrial networks. I don't know where it stands right now, but it'll likely add larger cells support. But it's not there yet for sure.
Based on my read of the summary of a couple Mbps per cell and focusing more on low bandwidth services like texting makes me suspect they're right at the limits of what the tech will allow. Since they're advertising compatibility with existing hardware, makes me suspect there is an extension somewhere in the standards that allows the timing window to be increased, or maybe there's some clever hackery going on, like only scheduling every other timing window and cutting bandwidth in half.
Which might work on the uplink, but on the downlink I think it's more complicated to support something like paging, where the UE is mostly idle, and only wakes up occasionally, that things don't get out of sync with the moving satellite.