"The coach needs to understand the general framework, limitations and capabilities of and within the game."

--- This is obvious. Also, I never said that strategy isn't a piece of the puzzle that coaches don't need. Strategy wins games and can be considered more important than mechanics in many cases. "You do not need to be D1+ to know this or even high ELO in general." You're right, you don't need to be D1+ to know high level strategy. However, as far as mechanics go, there is probably a correlation that the higher ladder level you are generally the better mechanics you have. If you have better mechanics than most, then you can probably catch mistakes that your players make over someone who does not have good mechanics.

"But it isn't the general's job to know how his soldiers swing their swords, only that they do so at the right time and place."

--- Why are you talking about generals when this is a conversation about coaches? Why are you trying to discredit the advantage that a high ELO coach brings to the table? Anyways, let's end your delusional thoughts on the subject and use your example of a general.

Scenario, you have two generals, GENERAL A whom is a highly experienced fighter (master-at-arms / war veteran / master strategist) and GENERAL B whom is a master strategist (equal level of GENERAL A) but has minor practical experience with weapons in their life. As a solider within each general's army, which general would you prefer to lead you into battle that required not only great strategy, but also the most advantageous use of the tools in your arsenal?

The question is rhetorical and the answer is obvious.

"Meaning SoloQ Elo/Individual Play Level =/= Coaching Ability"

---Once again, this is an obvious statement because inherently not every player has the capacity to be a good coach. ELO is not the sole definition of what makes a good coach for LoL. It is just a preferred characteristic of the coach we want because it would be beneficial to the team overall.