How would you suggest it get plumbed in? The JJ has a cleaner routing because it doesn't allow offboard dil to be plumbed in. The Fathom has you plug everything in so you know what you're breathing at any given time. Different priorities based on the types of diving that do it.
Don't know this for sure, but since Lundgren was really the one getting the JJ sorted it makes sense that not plugging offboard in was a priority of his since his real expertise is in deep wrecks where that isn't a priority. In contrast, Charlie does extreme long range cave diving where it's extremely important to be able to plug in stage bottles or deco bottles. For the vast majority of divers this doesn't matter at all, but if I NEED stage bottles to get out from where I'm going in a cave then I want to be able to plug them into the breather if the exit goes really sideways.
@divingx I wouldn't say breathable gas when diving hypoxic was the priority, it's much more the ability to plug in stage/deco bottles if an exit goes sideways. The fact that hypoxic mix is breathable is a perk, but since even 10% is breathable at your oxygen stop, it's definitely not the primary justification.