IIRC, in this case the FBI served up a "diagnostic" executable to users. Since they're using Tor browser, in general, simply enabling Javascript won't do anything to de-anonymize the user (unless the Feds had a TBB 0-day or something of the like). This executable obviously called out to FBI-controlled servers and provided a real IP.
Basically, the only people they ended up busting were people who for some reason decided it was a good idea to download and run an executable being served by their favorite CP site on the "darknet".
Basically, the only people they ended up busting were people who for some reason decided it was a good idea to download and run an executable being served by their favorite CP site on the "darknet".