Basically, I believe that when RJ wrote a scene where two channelers were interacting, his relative strength chart was either always present, or memorized with respect to the two channelers.

He was able to slip strength comparisons into casual conversations or random points of view effortlessly.

In contrast, Sanderson simply is not able to do this. He is focused on getting the plot to work out, getting his character development right, and making sure that the general tone of the story remains true to what went before.

Compared to all of that, direct strength comparisons are simply not a priority. In his answer to the strength issue, he said that in the notes somewhere he RECALLS that RJ has a strength scale of every single channeler.

The way in which Sanderson refers to this suggests that he hasn't really spent an aweful lot of time looking at it and that it is not really something he is an expert in.

In fact, he may not even KNOW which of Semirhage or Nynaeve is the stronger channeler, since it is irrelevent to the plot of tGS.

The example of one woman holding Semirhage's shield ties in perfectly with this. We know this is impossible.

It is a clear mistake due to Sanderson's lack of understanding of either the One Power strength gap between Semirhage and Dagian, for example, or the dynamics of holding a shield over a stronger channeler.

Heck, Nynaeve herself stated that no one sister could shield her like Berowin did. Hence the need for Berowin's talent.

And we know that Semirhage is AT LEAST an average strength female Forsaken, and therefore AT LEAST as strong as Nynaeve's full potential - which is much greater than Nynaeve's strength in CoS at the time when Berowin shielded her. Making the idea of one sister holding Semirhage's shield even more preposterous.