I measured 3 μA current during sleep using the following test setup:
Heltec CubeCell HTCC-AB01 board.
Only power (and antenna) connected to the board.
Powered from 2 alkaline AA batteries (3.1V) directly to the 3.3V/VDD pin.
I used the Basics/LowPower_WakeUpByTimer example sketch to put the board in and out of sleep and increased the timetosleep and timetowake values to 60000 (60 seconds) so I had some time to do the measurements (μCurrent Gold requires manual range switching).
Just to be sure I additionally disabled Vext (at the start of both OnSleep() and OnWakeup()).
I even had AT Support still enabled in the IDE (had forgotten to turn it off). Maybe the Radio.Sleep() call in the sketch also disables the AT stuff during sleep but that’s just a guess.