The focus is on the 'letter writing' because it is an example of how the Commission's work is being resisted.
It is just an example.
The idea (or the process) is meant to run like this, in three stages:-  The Commission (advisory) makes recommendations to the Pope; the Pope accepts those recommendations with which he agrees; the worldwide Church implements the agreed decisions.
That's the idea.
Marie Collins has seen the idea break down, with this - as one might think - completely innocuous recommendation (agreed by the Pope) that the CDF should acknowledge receipt of letters which it receives from abuse victims. For the CDF has refused to comply.
If the idea won't work with something as simple as a letter of acknowledgement, how will it work with anything more important?
The straw has broken the camel's back.
Like you, Cardinal Muller has tried to deflect the argument away - into other areas (wider areas, of course, and mostly irrelevant or tendentious).
He has failed. And so have you