As we all know that to make optimal profit here in cryptocurrency specifically in investing bitcoin is to buy low price for bitcoin and sell in higher price. This is why others think that holding is the best way to earn and in fact they are right. So for those who are still eyeing to invest in bitcoin this is a call for you to invest now while market price is still low.
The first two sentences you've said were correct.
There's no definite time to invest in bitcoin, everyone is free to invest and buy bitcoin as they wish as long as they have the capital to start buying.
Why not buy bitcoin because you can use it as a currency not an investment?
He forgot that there's a technology to support and being a currency itself. Everyone looks at bitcoin as an investment/asset so he wants to tell others that before it's price rapidly increase, start buying.
I advise, buy when they are ready.