The implication is that the syllable 'いさ' (also written as イサ) of 'いさぎよし' (also written as イサギヨシ) was pronounced with high pitch when the Zushoryo manuscript was compiled.
図書寮本の成立した当時、「いさぎよし」の「いさ」の部分を高く発音したことが分かる。