Should I avoid using personal pronouns in an abstract?
The requirement you have found refers to the text of the graphical abstract, not to the textual abstract. A graphical abstract is an image with little text: thus, it is reasonable to have stricter requirements, with respect to standard abstracts, on what is allowed or not.  

See also this page on graphical abstracts.

As for standard abstracts, I quickly checked a few author guides from IEEE, IOP and AIP and none of them seem against the usage of personal pronouns in abstracts. Also The Chicago Manual of Style, 16ed, does not mention anything about the usage of pronouns in abstracts. I've used the pronoun "we" in several abstracts and no one objected.

In general, the main requirement is that the abstract should be self-contained: no citations (if really necessary they should be written inline), or references to figures and tables.