What about a flag saying “Mark Mitchell for OPA Board?”

Or “Eat at Jake’s Diner?”

Flags can be signs.

noun
1.
a piece of cloth or similar material, typically oblong or square, attachable by one edge to a pole or rope and used as the symbol or emblem of a country or institution or as a decoration during public festivities.
"the American flag"

a flag saying "Mark Mitchell for OPA Board" would be a waste of money.  Especially since flags are normally used for easily recognizable symbols, and are not easily read as they are usually hanging down in an unreadable position.  The definition you referenced is for a flag, not a sign.  My definition of a sign would be considerably different.  I'm pretty sure most people would easily distinguish between a flag and a sign.

ye, there are flags out there used to promote info normally promoted on signs.  but that doesn't make those flags sign.