In the Tweet, Diane Hayes boldly claims what PIO means.

Shall we dissect that?

Public ...  pertaining to, or affecting a population or a community as a whole

Information:  knowledge communicated or received concerning a particular fact or circumstance

Officer: a person appointed or elected to some position of responsibility or authority in the government

Not all PIOs are and remain the same.  Most of the time, one is appointed to the position.  Many times that appointment is not desired.

Did you know that next to the death of a child, public speaking is one of the most feared tasks?

Nevertheless, not all PIOs have been trained in their role.  Many times the PIO is selected, especially in the private sector, because of their media experience.  While media experience is important, should it be the ONLY factor?  Sometimes the PIO is appointed to "fill a slot".  While the Incident Command System is important, some practice NIMS by appointing command and general staff regardless of talent.

Nonetheless, the Public Information Officer must "provide the RIGHT information, to the RIGHT people, at the RIGHT time, so that they can make the RIGHT decisions."

Twitter / @hayesPT: PIO stands for Public Information Officer ...: