You should first know that Blastoise is hiding statistics in this argument. He claims Jax's lower player base (pick rate) causes more winrate fluctuation, but then doesn't show any data to support that claim. He shows Ivern's graph, but not Jax's. Please note that Ivern's pickrate is dramatically worse than Jax's, by nearly 10 times.
It's also important to note that this argument is disingenuous. There should be a similar percent of players getting lower winrates on Riven as their are Jax. If, for example, 10% of Jax mains are statistic inaccuracies, then should also be roughly 10% of Riven mains who are statistical accuracies. It's the same elo/skill level, so statistical, inaccuracies should be similar. Blastoise only says Jax has this issue. That means the number of statistical inaccuracies doesn't seem to be the same. This means that, on average, Riven mains have a higher winrate than Jax players. That's why as you go into high and higher elo, like Master, Riven's winrate dramatically increases while Jax's stagnates.
There is also the huge issue with Blastoise's data not listing ELO. Jax is a lot stronger in low elo, while Riven is a lot stronger in high ELO. If we're talking game balance or design, we should be dealing with high elo. I can only presume this is all elos accounted for rather than high elo, which makes this graph look very favorable for Riven in terms of not looking as overpowered.