This label is commonly used by aro-spec people or others who are in or desire a QPR. While polyamory isn't inherently a romantic label polyplatonic can be useful as a way to say one is looking for a non-romantic polyamorous relationship.
Polyplatonic may also be used as the platonic equivent to polysexual. Under this definition one can be platonically attracted to many, but not necessarily all, genders.
The creator of the flag is unknown, though it has been used since at least August 29, 2015. Green was chosen as the opposite of red which is a romantic color, and purple was chosen to be the opposite of the yellow pi symbol on the polyamory flag. The other colors do not have meanings and were chosen to resemble the polyamory flag.