No, not true.
As others have said, the extremely hot ones are not the normal, but rather the exception. I have a eventer-turned-showjumper that excells in showjumping, and he is the best behaved horse I have ever had the pleasure of working with. He'll stand cross tied for hours if you need him to.
Also, I believe that some hot horses would NOT make good showjumpers. I had a OTTB gelding at one time, and he was your typical hotter thoroughbred, and he wasn't good with showjumping because of the mental stress/focus needed/all those colorful jumps to spook at!
I also agree with the previous answerer, because I own a little QH that loves to run barrels and other speed events, and always places in those events, but he also always gets ribbons in HUS classes, halter, showmanship, and english equitation classes.
Years of working with OTTBs, in hunter/jumpers and eventing