Neither orthodoxy nor truth divide into right corners and left corners and far-right corners and the like.  Maybe certain types would be more inclined to read this if someone they thought was "in their camp" had signed on.  (And seeing Fellay's name raised my suspicion of the document, perhaps unfairly.)  

But given the level of detail in the arguments it had to have been a chore to draft something that 40 signers could agree to without reservation.  If some people more affectively/associationally "Left" have a different take equally respectful of the Church's tradition of reason (as opposed to anti-intellectual dismissals in the style of Massimo Faggioli, which reduce to: "the signers don't meet some arbitrary standard of ecclesial rank therefore they don't count") then let the conversation begin and the truth prevail in the end.