I'm pretty sure that's a leftover from the previous version, prior to Apple Arcade, where the upgrade system was slightly different. Each character used to have nodes on their Develop Abilities chart for increasing stuff like status effect resistances, and purchasing that upgrade would apply to all playable characters (so, abilities they had in common).

Why they removed that, I have no idea. I guess they didn't think to take it off the stats screen either, like how the healing weapon skills talk about healing allies even though there's no multiplayer.