By your definition, if I use a set of doubles on a recreational dive, one of them is just a very big pony.
I sling a stage, a deco bottle, & on the very rare occasions I use a pony, I sling that too.
I use a stage to extend bottom time. I use a deco bottle to accelerate deco. I use a pony for those times when it all go's to pot on a recreational dive & I need a personal emergency gas supply. It's not how I mount it, it's about how I plan to use it, that changes the definition of the same slung bottle.
In a word, bollocks
If i'm on a single tank rec dive, & I sling a tank that I haven't planed to use for extending bottom time or to accelerate deco, but I have planed to forget about, (except to charge the reg occasionally), unless faced with some sort of emergency OOG situation, what is it?