I wanted to thank you for this feedback (adding a new "Recent Activity" sort order to forum lists) and let you know we're going to roll this new sort option out to the production server this evening.
Cautionary note: To make this work, we're having to re-index the site (which takes several hours). During that time, the sort order will, indeed, be out of whack.
We hope to have the indexing completed at the latest by Sunday afternoon and at the earliest by Saturday morning (PST).
Thanks again for your support.