Well..I guess you're right....lol A bow "form?" made with a 2x4 base with short pieces of wood sticking up from it to clamp wood slats to isn't a jig.
To answer your question....the 2x4 base wasn't rigid enough nor stable enough as it warped a bit over time.
From Wiki...."A jig's primary purpose is to provide repeatability, accuracy, and interchangeability in the manufacturing of products. A jig is often confused with a fixture; a fixture holds the work in a fixed location. A device that does both functions (holding the work and guiding a tool) is called a jig"
A caul is strip or block of wood used to distribute or direct clamping force....though I've heard many bowyers calling a piece of curved wood used to clamp/steam in recurve tips a caul.
Maybe a bow "form" is a gluing fixture?
I learnt me sumfin t'day! Though I'm still a little confused.