StoneyB has provided what I take to be the correct interpretation of the quotation. To his answer, I would only add that many of the remarks attributed to Yogi Berra as "Yogi-isms" have in common a kind of logical turmoil that prevents the statement from making sense if taken literally. For example, Berra is also noted for having said:
Nobody goes there anymore—it's too crowded.
I wish I had an answer to that because I'm tired of answering it.
and of course
I never said most of the things I said.
The logical impasse in the OP's quotation is that, if certain people really couldn't ever be told (that is, learn) anything they didn't already know, they couldn't learn anything, period. What you already know from the outset, you don't learn; and what you learn along the way, you didn't already know. So the literal logic of Berra's remark is fractured, but the intended sense of it is, as StoneyB says, that some people are very bad learners because they are extremely resistant to new ideas.