Diversifying your portoflio into different altcoins is such a bad advice that it doesn't even need further elaboration, but what i would like to hear from you is logic behind using different exchanges to "minimize risk"?

You are not supposed to store your crypto on exchanges and instead use them for what they are initially intended for (exchanging crypto) so as long as you withdraw your funds immediately after exchanging, it doesn't really matter whether you use 1 or 5 of them. Unless you are trading obscure shitcoins that can be usually found on smaller exchanges, but that's another thing and its not done due security reasons.