I don't understand the higher performance either. What I know as the significant advantage is that you can have one object in multiple lists.
Another advantage is smaller code size, as the compiler doesn't need to generate code for SinglyLinkedList(i32), SinglyLinkedList(User), and SinglyLinkedList(PointCloud). This could have a performance impact by making it more likely that code remains in the cache.
Usually if I have multiple lists holding something I have one that's the 'owner' and then the secondary data structures would have a non-owning pointer to it. Is that the case where the performance would be better with an intrusive list? My intuition would be that having multiple Node members would pollute the cache and not actually be a performance win but maybe it is still better off because it's all colocated? Seems like the kind of thing I'd have to benchmark to know since it would depend on the number of lists and the size of the actual data.
Since you shouldn't reach for a linked list as a default data structure modern hardware anyway, I actually do see how this change makes sense for Zig. Neat!