You're all saying "Never quite understood tremendous aversion to large position sizes" and "I never understood the concept of 'positioning'."
...meanwhile, the guy saying he has 40+% of his portfolio in Fairfax had his (previously) largest equity position go to zero last year.
The world is an unpredictable place, and diversification is protection against that unpredictability, not just protection against an investor's knowledge (or lack thereof) about a business. The more concentrated you are, the lower the probability events that you need to consider...because any sequence multiplied by 0 is 0.
If you have your entire portfolio in FFH, I hope (for instance) that you have modeled what would have happened in the depths of the COVID crisis, when FFH was seriously strained, if there had been a super-cat event and they had TRS swaps in their portfolio where they needed to post additional collateral at the same time. While I do hold a significant amount of Fairfax, I, personally, am not concentrated enough to care about the very, very low probability events.