I assume he meant "stable" in the sense that high-cap coins are less volatile than low-cap coins, which is true. But I would always expect ETH to be more volatile in both directions than BTC.....I've been using it as one of my gauges for overall altcoin sentiment for the past couple years.

Anyway, 85% is nothing out of the ordinary for an altcoin. Bitcoin has had bigger drops from its ATH in the past. 2014 was slightly worse than that.