What is the point of using 謂わば in this sentence?

This 謂わば literally means "if I say", but it is an idiom that means "so to speak". The sentence translates to "Between a joint and a joint, there is another joint, so to speak".

This should be a reference to the weird modeling of the characters in FF7 on PS1.

A human's joint (e.g. the elbow) is usually a thick part of the body, but as you can see, the characters in FF7 had remarkably thin elbows and shoulders. So I suppose this comment wanted to say "Between Cloud's two thin joints, there is another joint(-like thickening), so to speak". I don't think this is a very good joke, and perhaps most people did not understand this comment. You don't have to worry too much.