The proof in favor of this is precisely in the fact that O'Leary talks about "the top 7 cryptocurrencies" because that way the profit would be much higher than if only BTC was traded.
It betrays his ignorance of cryptocurrencies. Not only is there no such thing as "the top 7 cryptocurrencies", and the coins on that list will change markedly depending on who you speak to since everyone just shills whatever bags they are holding themselves, but diversifying across cryptocurrencies does not provide protection against large price changes as it might in other markets. The most likely outcome is that you simply amplify any losses, as if BTC crashes then altcoins all crash harder.Most of the time it's better to just stick to BTC, especially if one doesn't have the time or energy to follow what's going on with altcoins.
Most of the time it isn't even a case of following what is going on with altcoins. You could spend hours of your life researching altcoins, finding one which actually has experienced and active developers working on real development targeted towards a real problem (and there are very few which fulfill that criteria), and watch the value slowly bleed out, while some scammy centralized vaporware like XRP or EOS doubles in price overnight. Picking the right altcoin to bet on (if making more BTC is your primary concern) is more an exercise in the psychology of the masses than it is finding any real technological advancement. 
