First of all, the headline stating that "Liberals" urge Trudeau to break promise'... is misleading. Five Liberals did.  Not Liberals in general. 
Secondly, the promise that the previous election would be the last FPTP  election was clear and unequivocal. It is not a promise that can be dismissed since nothing has changed to justify it. 
Finally, the French run-off election system is a positive alternative since it ensures that no party gets elected without a majority. Even if people only get their second choice, they never get the worst choice unless they vote for it freely. 
That's democracy.