Come on Dre, are you being for real? What advantage would zilean get in a utility mage meta? How would it make sense at all for him to be popular then instead of other utility mages?
But what archtype is completely dependent on being able to pop a member of the opposing team, and becomes useless if they can't? Yep, assasins and burstmages, and who has the strongest "anti-pop" in the game? Zilean. This is especially true when the adcs are hypercarries, so if you used your head about it you'd realize, THIS IS LITERALLY THE BEST POSSIBLE META FOR ZILEAN.
Similarly kha and rengar have come into meta because of repeated lee sin, elise nerfs, and evelyn nerfs, similarly the AD casters fell out of play for jungle because lee and elise suffocated them and evelyn was much better out pulling out early ganks because wards don't work on her. Why'd rengar and kha come through as such strong picks when all but lee left the scene and lee was very dimished? Welp, the overall game shifted DRASTICALLY, the era of the hypercarry was upon us, and both rengar and kha were very good at assassination, while kha also provides poke.
As for season 3, 2 words "gold flow", the change in wraith positioning and the addition of wright, along with the lower spawn timers for jungle camps were a giant gamechanger for junglers because it meant that laners were no longer stealing half of their gold income reliably. This made bruisers a great deal stronger in the jungle even if they didn't hardcore snowball. This enhanced an already strong lee sin and made elise and eve really powerful. Over the course of the season people learned to invade better and tank junglers gradually fell off due to being too much of a liability.
See, here's the thing, it's not that champions don't go unnoticed and get discovered later, it's that those champs get discovered in metas which are strong for them, ex things that counter them get nerfed, the game encourages multithreat teams and they're a threat, the game encourages single threats and they're good at buffing or peeling for a single threat, unless they counter the existing meta in such a strong way as to warp the game around creating a new meta.
Point is, it makes no sense to say you knew a champion was that strong months ago unless you're arguing that he was this strong in every meta, both that he did well enough against the existing meta picks to justify presence in lane and that they provided enough strength (either of the types the meta advantages or enough simple raw power that they can be thrown into a team) to justify placement.
Top lane was a very different place months ago with very different picks, to argue that he was strong then because he's strong now is pure fallacy.
Not to mention the reason that jax worked in the meta is that none of them were actual counterpicks, the entire point of why he was discovered was that people realized that his counters were only counters early, and they generally didn't bully him enough to prevent him from scaling up, especially given that to bully him you need to be aggro and he has great gank support.
So ya, TL;DR: Powerful now doesn't mean he was always powerful and none of those cases suggest that characters are just powerful in the aether. Yes characters have to be discovered but they still need to be good in or against the meta.
edit:
Tank junglers are plenty viable below, diamond I'd guess. The entire problem is they have pretty static paths, are slow early in the jungle, and get destroyed in duels.
Also, you're playing against a tank jungler, what exactly is rammus' plan for living in your jungle, roll around you threateningly?
