It would be interesting to learn a little bit more about what this means. X86 "supports recursion" in the sense that it has a stack and the push, call, ret, syscall, etc. instructions make use of it.
I read the paper, which includes a forward reference to Figure 1 (the system architecture). When I got to the end of the paper, it says that Figure 1 is "TO BE SENT SEPARATELY BY MAIL, TOGETHER WITH A PHOTO OF ONE OF THE CUSTOM CHIPS". Oh well.
It supported recursion in its microcode which is very unusual.