My statement was a conditional one, as you can see from the words "If this is true". You can disagree with the hypothesis this is based on, but that does not invalidate the conditional conclusion.

ZachPruckowski and RyanMcGreal seem to be claiming that variation in teaching methods has little effect on achievement, and demographics/non-school factors seem to be the major cause of variations in achievement. If teaching is not a major cause of achievement, we should not waste time and money on it. Similarly, we should not waste time and money on shark repelling sticks or homeopathy.

But I should point out that the anecdote you repeat seems to imply that teachers don't have a particularly large effect on achievement. As you say, "even there you had problem kids who refused to make any effort" and you had "the ones willing to read the book for its own sake". The teacher won't help either of these categories of student very much. Supposing, for the sake of argument, that these two types of students make up 2/3 of the class, and a good teacher can improve the grades of receptive students by 20% over a crappy one, then teacher quality only accounts for 6% of the achievement of the class.