Author mentions, but doesn't focus on, 'work being too challenging/not challenging enough'. I wrote a fair bit about it here (with a slightly different focus as name suggests, but I go over original research first) https://incentiveassemblage.substack.com/p/why-is-nobody-ser.... I'm not sure why 'challenge level' is less focused on compared to lack of interruptions - both seem about equally demanding to environment including manages and take similar amount of work to adjust.

Either way, to save you a click, Csikszenthmihalyi research wasn't mainly about cognitive load, because we already had a fair bit of research on cognitive load. It seems insufficient (although I do have my reservations), but addition of complexity of the task and w/e additional issues are happening is pretty solid predictor* of performance. Challenge/skill 'graph' presented can be reinterpreted with challenge/skill on X axis and a parallel flat line above it. Even better, and empirically supported, graph can be seen in first image in the post I linked, but it is a bit much to paint with words.

Flow research is cool, but there are more simple and actionable tools.

*Observant reader may notice that this is because of lack of units, but we do have physiological indicators if one desires to monitor them.