I think you miss the entire earmark of the average tier list that is designed by people (especially by myself) considering that the maximum I ever see a party is ever at 4, and most of the time I would argue 2-3 is a lot more efficient. You also will notice I point out things such as Experience gains fairly often in some of my arguments (though not as of late, only because I've been tied by other means). One of those times I mentioned how Cranidos's Erratic EXP gain can be really detrimental to it sometimes (though it can be a benefit) whereas I also will argue that there are times where Gyarados's Slow Experience Gain can really hold it back and, sometimes, keep it from getting up to the critical levels it needs to get. But, at the same time, if it soloes at a certain point in the game (which some can) there's also where EXP gain has a slightly lowered playing field over a Pokemon so long as the stats and move distribution back up the Pokemon. Typing too, to a lesser extent.The main issue I have with the lists is that they serve no purpose.
If they're supposed to be a guide towards an efficient run-through, there's nothing much to measure efficiency by; it's subjected to whatever arbitrary restrictions the writer assumes "sensible" from the view of the conjured-up Everyman, such as trading for Kadabra or Kingdra being plausible, but not for Seel, Houndour or Kangaskhan, or for a Metal Coat before Falkner; and that the Average Party shall be composed of 4-6 pokémon, not 1-3, and they are all being kept at the same level, no matter their EXP growth curves, due to the allure of superficial symmetry.
The solo play is an interesting theory, but the problem is that it requires a specific setup and usually only has a couple Pokemon that really benefit from it. Even then, it doesn't necessarily rank them accurately as a whole by solo play as some of these Pokemon listed also do more than fine on their own in solo play (Totodile in GSC for example, which is already in S). Solo play has skewered results as can be shown in games like Fire Emblem where it's usually more efficient to have at least a second or third Pokemon. Knowing one of the original pioneers of this list, he certainly did not assume all 6 Pokemon being used and probably would argue that 4 was a bit too much in RBY as an example. Even though he isn't around IOS and I have discussed at great lengths about solo play with things such as Tailow in HGSS and how Poplio is also a highly used solo candidate (at the time) for SM. But at best it could argue Tailow higher with Poplio... well... already Top Tier. It's something that could be used to help strengthen a Pokemon higher for example.
If a tier list is ever assuming more than 4 Pokemon, the chances are it's probably incorporate HM Slaves, something that has had a significantly lower ranking throughout the years since there is arguably a lot of room for HMs (it just compresses in situations where you have a party of 2-3 and you catch a legendary) and some of your party is arguably having some of the benefits like Surf or Fly used anyway (Fly may be weaker than Drill Peck, but Fly has a wider distribution than Drill Peck). A person in a tier list may showcase off a party of 6, but that does not mean a tier list should ever follow in an example that a person uses all 6 slots for Pokemon for battle.