I just did a quick test on one of my servers with a E5-2696v3 CPU that has 18 cores and hyper-threading disabled with the “nosmt” kernel option. In that test htop displays 18 cores running and another 18 in “offline” state just like you see. It’s unusual as most programs just display it as a system with 18 cores. So it looks like in your case you have hyper-threading disabled, which is a good thing IMHO, it gives less than 10% performance benefit at the best of times while increasing RAM use and making you more vulnerable to some security problems.
Given the stated aims of Purism regarding security etc having HT disabled by default seems like a good choice.