This particular part of "physicist's history of physics" about quantization and Planck's work, promulgated by some sources, is well-known to be a false account of history and motivations, and has been criticized in mainstream literature before, e.g. by Helge Kragh [1] (and probably by many others). The present authors apparently are not aware of this, which makes me suspicious that they did not do their homework on this topic...

[1] https://dept.math.lsa.umich.edu/~krasny/math156_article_plan...