And that's where I can't really argue anything. All I've got is the idea that Piranha Plant was planned to either be a base roster character, or separate from the DLC pass characters. He was already in development before Ultimate released for some time, so his name in the files is literally his name. This is kind of moving the goalposts, but Plant is undeniably separate from the Fighter's pass, so who's to say the codename could not reflect this? I just question all of this because again, "jack" is not a direct reference to Joker, so why would he be the odd one out?
Edit: Others vocalized my Plant retort faster than I did, but you understand.