The library library(nb_set) defines non-backtrackable sets, implemented as binary trees. The documentation comments clearly say that a hashtable is used and from a quick glance, that information is correct. It also mentions the change explicitly:
The original version of this library used binary trees which provides immediate ordering. As the trees were not balanced, performance could get really poor. The complexity of balancing trees using non-backtrackable primitives is too high.
This is just the most blatantly obsolete piece of information in this manual section; the rest has also been visibly updated. Refer to library(nb_set) directly till this manual is updated.