I also found it amusing that they assumed the tier lists revolved around optimization stunts, because really any class can reach Tier 1 with sufficient optimization.
The tier lists only operate under the assumption that you know your class and how to play it, not that you are doing some kind of optimal game-breaking tech. What makes 9th level prepared casters Tier 1, for instance, isn't the fact that they get 9th level spells, but the fact that they have an extremely expansive suite of solutions to virtually any sort of problem and they get those spells earlier than anyone. At most they need 1 day to prepare all the spells they need (or also, in the Druid's case, to dismiss and recruit a new animal companion with a new build) and obtain a tailored solution to utterly trivialize the problem the party is facing. 6th level casters typically do not have full access to their spell lists without stunts or have more limited spell lists to draw from (Warpriests for instance do not get domain spells, Magus has a more limited list and is penalized for trying to get equal-level Wizard spells, Alchemist has a rather limited list as well), they tend to get their spells a bit later to boot, and they're less likely to be capable of becoming fortresses unto themselves who auto-solve any problem in a single spellcast or a few spellcasts if it's a non-combat task.
Wizards on the other hand can easily shapeshift into elementals with large temporary hitpoints while retaining spellcasting capability, utterly subdue entire encounters of enemies in a single spell, summon monsters to annihilate whatever stands in their way with extreme prejudice, place contingencies to teleport themselves away from chance of death, make simulacrums and undead armies, create walls of iron and fabricate them into armaments fit to supply a regiment, divine their enemies' every secret, read people's minds, mind control their enemies, thrive underwater or in outer space, teleport around with ease, and the list just goes on and we haven't even touched the level 7+ spells. Druids can shapeshift into an extreme variety of potent forms while retaining spellcasting ability and collect +15 AC without any penalties just by wearing a +1 Wild Dragonhide Full Plate and then a +1 Wild Tower Shield (since in wild shape they are no longer wearing them but get their AC bonuses anyway, thanks to the wild magic property - oh, and this is not counting things like Magic Vestment or Barkskin), can fly and earthglide through dungeons with trivial ease, can abuse wild shape for all manner of poisons and other abilities with extreme DCs, can dismiss and acquire new animal companions for free by doing a 24 hour ritual and then proceed to build new animal companions in different ways for their encounters (which you can take very far), can summon hordes of beasts/elementals/etc. to murder whatever is in the druid's path (alongside the druid's own animal companion), summon some satyrs to dominate social situations, create entire fortresses with stone shape, move earth, and wall of stone, arm large numbers of people with ironwood and wood shape spells, scry out the positions of their enemies, interrogate animals, plants, even stone for secrets, cripple armies with winds, thrive in water and outer space (or even buried in stone), teleport massive distances through trees and plants, and loads more before we break into level 7+ spells. And neither of these two examples have even left Pathfinder Core or involved any feats (aside from Natural Spell, although there are a number of wild shape forms that can cast spells without that) or magic items.
Now a sufficiently well-played 6th level caster can readily reach T2 or even break into T1, but that isn't the norm. Reaching T2 generally involves some actual mastery of your class and cleverness (or rods of Dazing Spell metamagic, as ways to abuse this will swiftly make themselves apparent to you once you go down this road). And with game-breaking intent really anything can become T1, the most basic example being using Use Magic Device to activate, say, scrolls above your level (like Summon Monster IX if you feel like auto-winning combat, or Planar Binding, etc.), or using Magic Craftsman + Craft Wondrous Item feats with ranks in Craft (Candle) and making Candles of Invocation to obtain Gate spells for less than half price long before anyone should be casting 9th level spells (this is possible even in Core-only games). Another basic way to achieve T1 is to simply use a Leadership feat to get a T1 cohort. Even a Commoner can become Tier 3 simply through a sufficiently clever combination of feat selection, trait selection, skill usage, racials, and/or investing into a wide array of handy magical and non-magical items to give you a plethora of solutions to various forms of problems. Depending on how far you take this, you can enter T2 or T1 as a Commoner as well (even though the class does nothing for you). Highly clever and resourceful players tend to excel in just about any class even when they're not doing game-breaking tech. But I do not list these classes as T1, etc. simply because you can do this.