I use it every day and, to be honest, I have zero idea if it is indicated or true and never considered the difference. All I know is, if I assign you a Mach, I am expecting that to be your indicated Mach which, if I had to take a guess, would be what is reported to me in the ADS-C report. I will take a look at the reports my next shift to see if it says.
But generally I treat it exactly like when I assign indicated airspeeds when I work a low altitude radar. When I tell a pilot to maintain 280 kts, I expect that to be indicated. When I tell a pilot to maintain Mach 80, domestic or non-radar oceanic, I expect that to be indicated. Only time I assign True is for a true airspeed, not Mach, for low and slow aircraft in Oceanic.
But again, I have no idea what the report gives but based on almost never getting reports that have Mach numbers much different from what I assign, I would guess indicated gets reported.