They're secret in the sense that he had other stuff he was supposed to be working on (it sounds like he has his writing planned out years in advance with all the unfinished series he's juggling), but because he's a writing maniac he got bored and/or needed a break from his schedule, so he wrote...
I signed up for the physical book/box/whatever it is! I only did ebooks/audiobooks for the other secret projects and ended up buying two of the physical books after the fact anyway, so figured I'd just go ahead and get a head start on that this time haha.