my guess is the equations to determine thrust are for some reason easier to formulate with the mach number instead of true airspeed for example.
with all the other data it doesn't really matter if the EEC gets the TAS or the mach number as the two can easily be calculated from the the other.
i found this
11.6 Performance of Jet Engines
and it seems the thermodynamic people like to calcuclate thrust based on mach number.
so as i said there are calculations to be done and which set of parameters is used to do these calculations does not matter as long as you have "enough" of the parameters as the others can be calculated from them. as the ADIRU calculates all parameters anyway i guess the EEC can choose which parameters to use.
what i could also imagine is that if the mach number gets too high in front of the engine there are aerodynamic problems so the engine needs to reduce power so the inlet stream remains laminar. but maybe someone who really understands these things will chime in.
may i recommend you the 120 page or so concorde thread there are quite some posts on engine operation there. there might me hints in there, but it's quite a long read
