I have a copy of the relevant Navtech charts which the crew used, and the only mention of 'M' and possible pilot confusion is on page 10-6 dated 06 March 2013 which says "Exercise caution when taxiing on TWY B due to confusion with apron M."
This is a small text paragragh on a page of text - ie not next to the aerodrome taxi/ground chart. Of the taxi/ground chart page 10-2 there is nothing to highlight any restrictions on 'M'.
Referring to documents other than those the crew used on the day does not, imho, add anything to understanding their behaviour, more over it only goes to point out the charting defficiences.
Note: it does refer to 'apron M', although to all intents and purposes it is indistinguisable to a normal taxiway.