According to him, his fear is that if he sells it during the next bull run, he will be taxed, and he doesn't want that.
Bull runs end with a short-living peak and a sharp fall. If you sell in the bull market, even after paying taxes, you will have more money than the average value of your coins during the bull+bear market.
To illustrate, look at the 2020-2023 period, what's the average price? I'm not going to look it up, let's just say it's $40k. But if you sold at $60k and paid 20% tax, you'd have $48k.