In TortoiseHg 1.7, I could point Hg Repository Explorer at file and see a list of versions. Selecting 2 versions and right-clicking allowed me to launch my favorite Visual Diff tool to display their differences.
I recently upgraded to TortoiseHg 2.0.3 and can't find a way to visually diff anything except the working file with the tip version. "Revision History" shows a list similar to the old Repo Explorer, but doesn't seem to have a way to lanch a visual diff. Has this featured been removed?
I'm pretty sure you can do this diff in TortoiseHg v2.0.3 - try selecting one revision by clicking and then CTRL-clicking on another revision and selecting "Visual Diff...".
I can't test this right now because I have to use v1.1.7 for unrelated reasons, but during my time working with v2.0.X I was able to do this. The bitbucket link you included looks like it's for a button to perform the visual diff, not the underlying feature.
NOTE: I was able to get a co-worker to test this, it does work (at least in some fashion) in TortoiseHg v2.0.3