The Activity column reflects the latest event in the topic, which means the latest event that affected any of the posts in the topic.
In this case, we're seeing a bunch of system edits like this one in this post:
If you click on the pen icon, you'll see the reason for the edit:
That edit is done by this automated script I wrote, which corrects formatting after the migration from the old forum. It processes about 500 messages per day, in order to not flood the authors of the posts with emails notifying them of this system edit.
We're now at post #3800 of 5700, so we'll see this kind of activity for the next 4 days. After this post-processing is done, the Activity column will only reflect real replies or edits.