One of our founders bought BTC early 2016 and sold @ $19.5k in 2018, and then bought again in 2020 when it was $4.8k

if you are curious, you can find some of his posts about that on Twitter
So you are claiming that he sold his coins when the prices reached ATH and then again purchased them, when the prices had bottomed out. Theoretically, this is the best scenario which you can imagine. But in reality, it is almost impossible to do this. We never know when the peak prices will be reached and the same goes for the lowest level as well.
Yes, it is the case. Yet the best scenario would be buying it not in 2016, but even earlier, but yes - from 2016 to ATH, and then buying again this year, is still decent ROI. And no, its not impossible actually if you don't simply trade, or HODL forever, but rather analyze markets, trends, hypes, news (including fake ones - what has been pushed to masses), mining activity, etc., etc. Over time it gets natural without too much efforts.