I agree with Cheezmo and AzoreanGigolo. In this instance “lean on” is used in a figurative sense meaning “depend upon” (see the entry at wiktionary), rather than a literal sense, such as in “lean on a chair”.
© Short prepositions (three letters or less): as, at, by, for, in, of, on, to, but, cum, mid, off, per, qua, re, up, via – except when used as adverbs or as an inseparable part of a verb (as in “Plug In Baby” or “Shine On You Crazy Diamond”).
To me the exception in 2© of the guidelines is not entirely clear. Is “an inseparable part of a verb” generally interpreted to mean “part of a phrasal verb” ?
This is what several editors have implied to me without stating it outright.