I had not heard of Poly/ML or Isabelle/ML prior to reading this paper. I tried to do a bit of background research to get an idea for it, but I may have gotten some details wrong. If I made any mistakes below I will be happy to correct them.
Poly/ML is a full implementation of Standard ML originally developed by David Matthews. Larry Paulson was an early adopter and implemented the Isabelle theorem prover. Isabelle/ML is the implementation of Isabelle on Poly/ML. Poly/ML was originally implemented with a single threaded run time system (RTS). With the ubiquity of multicore machines the RTS was modified to support threading, the garbage collector was modified and threading APIs were introduced. Finally, the modifications were tested in Isabelle. This paper is relevant to the frequent debates that occur on the Ocaml mailing list. Ocaml currently has no support for parallelism and this deficiency is frequently brought up as a negative.