Emily Bowman writes:
Whatever he meant, nothing about split infinitives is in my 1918 original copy of Strunk's rules,
I'm not sure what you mean by "copy of rules", but it's mentioned couple of times in the book. I'm referring to this passage, from http://www.gutenberg.org/files/37134/37134-h/37134-h.htm#Page_36 (you'll need to scroll down a ways):
Split Infinitive. There is precedent from the fourteenth century downward for interposing an adverb between to and the infinitive which it governs, but the construction is in disfavor and is avoided by nearly all careful writers.
The Gutenberg Project version claims to be the same as the 1918 version, except for the differences between typesetting of the time and a very small number of typo corrections.