Author here. Let's assume you cannot got short (which you can't in most crypto markets) and can only make money when the market goes up. The best you can do is avoid losing money when the market goes down.

But there are no pure market downturns. On a daily scale the market may be down, but that does not mean that on a millisecond or second-scale you will only see downward movement. There is just an overall downtrend, but there is still almost the same upward movement to make money. For HFT systems, it really doesn't matter if the market, on a daily scale, goes up or down. There is no difference.

In fact, on many markets the system does better in downward trends. Probably because there is more liquidity on that side of the book, a bias that may come from certain market participants.