Honestly, I can't see how someone who follows all these rules couldn't at least get to plat.
What else would they be missing?
Well I mean...It's a MOBA, you know? There are about 100 other moving parts that are not encompassed with 9 bullet points. Many (most?) may not be within the control of a single player. Just a few general examples of additional issues of importance:
- Dealing with snowball champs. Or better yet, coming back against a fed snowball champ.
- Knowing when to take a team fight (this includes important item timings, if your solo laners have good leads and can group, etc).
- Knowing when taking 5v5's is a losing strategy for your team comp (eg, the enemy comp has malphite + Amumu + MF) and you need to split push.
- Knowing when to use your summoners (especially TP as offlane).
- Knowing when diving a tower is or is not worth it.
- Map vision (this includes ward control with sweepers and vision advantage with volume of wards)
- Ward placement strats (when to ward defensively, aggressively, around objectives, etc)
- Knowing enemy ability ranges (I know I've been caught a few times thinking I was out of range of Malph ult when apparently it goes half way across the damn screen lol).
- Team composition basic concepts (ie when you need front line, initiators, disengage, poke, etc)
- Farming under pressure (this includes concepts like freezing the lane, letting lane push near your tower, pushing the enemy under their tower, farming against high aggro supports like Blitz, Zyra, Brand, Leona)
- Approximate enemy neutral spawn times (valuable for the first couple of red/blue spawns, as you can reasonably well predict where the enemy jg is based on where they started).
- Picking what you're good at instead of overthinking your pick. This is an important but distinct variant of picking "what you can play". Sometimes people go out of their way to niche pick something they are "decent" with to simply try to win laning phase. But the game doesn't end in the first 10 minutes, and if you've put yourself on something that probably can't carry late (eg. you picked Teemo to counter Nasus), you may wish you had picked something you were stronger at.
- Mid land and jungle rotation frequency and effectiveness (when to gank a lane when a given mid/jg champ, when to provide relief, when to help push a tower vs defending the mid tower and challenging enemy mid farm, etc).
...and you know what? Sometimes even with all that information, you can still simply get outplayed. Because while picking from among your best champs and understanding the lane and enemy comp you're dealing with, you're not playing against bots: the enemy champ across from you likely picked from among their best too. Sometimes that gold player with 300 matches on his favorite champ is enough to outlane that plat player on one of his decent champs.
And sometimes, as we all know, you can do your best Atlas impression and still not be able to win games because this is a team sport; not golf or tennis. The other 4 teammates need to be able to do at least some things right along the way, even if it's just posturing but not feeding while you split push.
In my last game, for example, I was able to carry an Anivia that started 0/8 I believe. She just kept feeding LeBlanc and even after strongly encouraging her to buy an Abyssal, you'll note that she never bought a single MR item. Had it not been for the fact that I really know how to play Jayce, we probably get rolled.
This match highlights another point: you can do all 9 of those bullet points (and all of mine as well), but what happens when your 4 teammates don't? Anivia's MR items never appeared against a LEBLANC, never mind the Sona. Olaf was 0/4 this season on the champ with a KDA of 2 didn't get armor until the game was almost over...while adding no more damage than support Morgana who actually bought 0 offensive items. You can only control your items and actions; we can't buy items, play other people's champs, or pick champs for them. And in a team game, that means some matches are going to be losses whether you can do anything about it or not. You can encourage people to pick their best, but when someone decides they want to pick a champ they're 0/4 lifetime on, the results tend to be exactly as expected.
I could go on and on. So while the list of 9 points is a solid baseline of information, suggesting that information is all that separates diamond from everything below it is at best specious. Basic mechanics, situational understanding, decision-making, ability to handle pressure and make the right play fast, the 9 you listed, the dozen or so I listed (plus all the sub-topics they entail), and probably 50 additional things that neither list covers all have their fair share of impact on your elo.
