The fact that Cardinal Burke did not sign it, may be due to his view of the situation being different , as someone has already mentioned on the thread. He may not have signed it because it would admit that the Pope had fallen into heresy, and that Burke believes that Francis would automatically lose his office.
So it shows that there is not one remedy for dealing with a Pope who teaches heresy. There are differing views, because the Church doesn't not have a set teaching on it.
The Rorate article says that many Catholics, even trads will, belittle this effort. Well, if so, then it's just being realistic, since it's unlikely that Francis will be too worried about it. The Rorate article also says that this is just a first step of a theological nature, and that it will lead, God willing, to an initiative a canonical nature from those who have a mandate to act. But who are those who have a mandate to act? Rorate doesn't say. There have been no Cardinals or even bishops who have backed this effort by contributing, that we know of, to the writing of the letter. Bishop Fellay signed it, but that's not the same thing as helping to write it.
It seems to me, that unless there is an effort by bishops and cardinals, then this effort will not go anywhere. Priests, educators, and laymen don't really have much authority to really do anything that will stick. Unless they are hoping to get the bishops and cardinals motivated to do something with this letter. I doubt that will happen, but I could be wrong.
I suppose it's a good thing that Bishop Fellay signed it, but it would have been better if he had confronted Francis directly, as he should have.