You are missing the point. It is stated in the book and it's clear as day. Who are YOU to interpret it to fit your argument? You can't pick and choose what you want to interpet, that's not logical debating.
I agree with Adas... It was a statement that could refer to Yoda's conviction. Until it is defined as strength in the substansive sense(ie., Yoda was the most potent Jedi force user ever) , I see no reason to interpret it as such. It could just as easily mean strength of convictions. As such, it is not an intelligent point to bring up to support an argument.
Besides, there is a flaw in the quote anyways. It fails to regard future generations of Jedi such as Luke, and as such should be retconned. The quotation itself is never defined to include only those Jedi before the PT.