There is no rule stating that a holster of any kind is required other than for the proficiency test.
" Concealed" is the requirement.
Concealed is defined in section 15-12 of C.O.P. Rules as
"Concealed" means entirely hidden from view of the public and not discerned by ordinary observation, even if covered by clothing, such that  reasonable person without law enforcement training would be unable to detect the presence of a firearm"
A "Sneaky Pete" is not technically a holster and neither is a handbag, purse, slingbag, backpack etc...

To the uninitiated, a Sneaky Pete holster looks like a case for electronics (cell phone, multimeter, or medical supplies).  They sell a variety of "covert" models to make them less likely to attract attention as a firearm holster.

Meh, would still not consider it a holster just as I would not consider a ComfortTac fanny pack  a holster.
Just cuz it identity's as a holster does not make it one