I asked Sarah Davis on the (official) SugarMate group what she is using as an uploader; she is the only respondent on that group who asserts use of NightScout. The other group is private; we’ll see if they let me join.
Obviously other people, including people on FUD, are using NightScout and I assume some of them are using NightScout with FireFly G6 transmitters (which means any recent G6 transmitter. NightScout’s web page implies, well, pretty much states, that Dexcom Share is the only way to work with the recent G6 transmitters, but elsewhere @docslotnick says that recent versions of xDrip+ work just fine:
Also the Spike web pages don’t seem to mention any issues.
So far as I can see NightScout and SugarMate are in the same position; they both get real-time data from Dexcom Share. SugarMate, being part of Tandem Diabetes, should have a really good relationship with Dexcom, but corporations are weird. Maybe NightScout have found some way of integrating the Dexcom Web/REST API?
There’s nothing magic, in the xDrip+ sense, in what NightScout do:
https://nightscout.github.io/uploader/setup/#dexcom
So that just adds log-in information for Dexcom Share to the NightScout code running on the internet inside Heroku. That (NightScout) code is available for anyone to read (I think; it’s a while since I tried Heroku.)