Interesting perspective, hadn't really thought about it that way before. Maybe I personally struggle more with avoiding stray hits from at kill percents more than I do with . With the frog, I feel like avoiding dtilt/dash attack into fair/upsmash until I get out of those confirm percents is most of the battle to fighting against him and that hasn't been too hard to do (with stray fairs, uair juggles and dragdown jab locks and tech chases being the other main things to look out for), whereas arsene-less Joker can win neutral and still go deep for edgeguards or opt to ledge trap reasonably well by a skilled player even online from my experience. He definitely struggles more with killing without Arsene, although I haven't experienced it being so severe as to equate it to sheik syndrome.As someone who plays both characters, I can say that has more consistent kill power and kill confirms online than does. Arsene definitely helps Joker when it's up, but he suffers from Shiek syndrome pretty badly when it's not. You'd be surprised how much easier it is to kill with the frog off of a stray Fair, Nair or Ditlt than it is to get reads with Joker. Plus, the two characters play similarly enough that any precision from Joker's play translates nicely to Greninja as well.
And yea, I totally understand why Leo would pick up a character like him for the reasons you and I appear to agree on. Leo is Leo, so he is still able to pull it off, but him not playing online when requires similar precision that generally isn't realistic to have online was intriguing to me.