I agree with this 100%. It is so often the case that once the BTC surge starts to slow down, then money pours into alts, and they increase as well. And also if you look at the charts for the huge bear market, you can see that alts started to drop in early January 2018... but you will also see that BTC peaked and started to drop shortly before that. BTC price movements can often be very useful in predicting how alts will move.
That's because the movement of bitcoin does affect market price movements, including the altcoin price. after prices have risen and returned to stable, etheruem and altcoin usually begin to follow growth slowly. but if there is a decline in bitcoin, with the altcoin market also following the decline.