I believe the footnote statement is quite simply not true.

Quoting from Edwin G. Pulleybank's “Outline of Classical Chinese Grammar”:

“Judging by the use on the oracle bones in which … 朕 refer (almost) exclusively to the king himself while 我 refers to Shang collectively, the original distinction was between singular and plural. By the time of the early Zhou bronze inscriptions, the distinction was breaking down and it was becoming possible to use 我 as singular 'I'”

“Preclassical … 朕 [is] mostly possessive, occasionally nominative. [It is] very rare in texts of the classical period. The First Emperor of Qin chose as 朕 as a special first person pronoun for his own use and thereafter it survived through imperial times as a way for the emperor to refer to himself.”

So basically 我 as a personal pronoun goes back to the earliest known written Chinese texts, though then the meaning might have been closer to “We” than to “I”.