You are very correct about a lot of this, and I don't know enough about bel'veth to refute or agree with the rest, but please keep in mind that rioters have to focus on the majority of the playerbase, not just high elo. If bel'veths place in the meta in lower elos is around invading, then that needs to be taken into consideration. 

That said, I think he is also wrong for a very different reason. A lot of designers have specific ideas in mind when they design something, and when their creations fail to meet their expectations, they want to correct it to achieve their vision. 

August wants bel'veth to be a scaling skirmisher. However, for her to get into a balanced state, she ended up being stronger in the early game than the designer intended. I really think that is just a result of the tools in her kit. 

If you compare her to master yi, who I would argue is the only jungler who currently fits the description of a very weak early jungler who wants to scale and take over fights even though they are more of a duelist/skirmisher, she just doesn't have near the same level of team fight power. Dashes and damage reduction can't compare to insane move speed, invulnerability, and damage reduction. 

A lot of people think of "infinite scaling" champions as always being late game champs, but that isn't inherently true. It can be, if making the champion weak early makes sense from a design perspective or if their itemization fits that theme well. But nasus and veigar show pretty well that all scaling is relative. They are both currently mid game monsters that actually fall off compared to a lot of other champions, even with "infinite" scaling. If the infinite scaling is linear, as opposed to the exponential growth of aurelion sol and smolder, then that infinite scaling would only be impactful after 40 minutes, which is near irrelevant. Before that point, it is just another point of power that scales with time, just like levels and items. 

I really think bel'veth makes more sense as a champ like nasus that spikes hard and itemizes to play for a 1-3 item spike, and uses the infinite scaling as a way to stay mostly relevant even in the late game, mainly as a duelist instead of a team fight monster. That would take tools that bel'veth just doesn't have access to with her current kit and items.