It will work, however I don't know if the "finished product" you'll be left with has been updated to MR1. Perhaps someone else here can answer that.
In a very basic nutshell, the root process flashes an OLD software version on your TB, allowing the bootloader to be unlocked. Once that's done, you then re-install a new OS over top of it. What OS version you currently have doesn't matter all that much.
If you use the autoroot version (same procedure, with the adb commands scripted to make things easier), you will be left with a rooted version of MR1 (the version you posted in the OP). I'm not sure what version the manual procedure you linked to will leave you with, but once it's done, you can always install any ROM you want anyway!
http://forum.androidcentral.com/thu...unroot-windows-updated-5-20-w-mr1-update.html