Game Knowledge requirements do not mean a champion has a high skill ceiling

If that were the case, then all champions have a high skill ceiling. Skill ceiling addresses mechanics. Game knowledge directs how those mechanics are used.

For example, Syndra has a very low skill ceiling and floor. She's very easy to use and very easy to master. The majority of her kit is maximized, like every other champion, with proper decision making and game knowledge.

Ezreal requires both great game knowledge and has a high skill ceiling to be used effectively. Without being decent at both or excelling at one will you be able to use him well.

It's too common an argument for players to use and not actually understand.

Aphelios does not have a high skill ceiling. ADCs generally have higher skill ceilings than the rest of the champion pool simply because their kits have less safety mechanics in the instances of major flukes and the nature of their design requiring them to position/orb walk/kite effectively. For a champion who should have the or one of the highest skill ceiling in the game as quoted from Riot, he has the same skill requirements as Jinx and less mechanical requirements than Kog. He barely holds a candle to Ezreal/Vayne/Kalista/Kai'Sa/Draven/Xayah as an ADC.

And as an ADC, he still holds less mechanical requirements to Azir/Riven/Yasuo in terms of ceiling.

Theoretically, Aphelios should have a steep learning curve with exponential growth at certain steps with his winrate, with a slow but steady growth afterwards. This often seen in the highest of mechanical champions (Riven, Azir, Vayne) to name a few. However, his weapon change is less mechanical and more of his own minigame. I believe the oversight of his kit is thinking his weapon change gimmick will be such a hard thing to master but it's actually related to game knowledge more. "Here are my periods and weaknesses to this comp. Here are my general weapons which would always be good."

I find myself doing 1K+ damage per minute with Aphelios easily because he's overcompensated for this oversight. Mechanical strength is not determined by the opportunities you have, but how well you can potentially use their opportunities. And for Aphelios, his paths to such opportunities are very clear cut. For other champions like Azir or Xayah, theirs are not due to how much could potentially happen.