I just tried to install. It installed fine, it grabbed only ONE post. What I noticed could help answer this.
I created a new category on my site (abuse-of-power) and created a new blog post to let everyone know. Then I back-tagged all other posts that are related to this new category. The ONLY post the plug-in grabbed was the original post I created. I suspect it is related to this.
Now, the plugin also grabbed other categories but I don't know how/why it grabbed those categories. I initially did not ask for those, I explicitly used the /feed/?tag=abuse-of-power in the RSS feed line. Oddly, it grabbed other tags that were related (i.e. freepress, freedom...) but did not grab any posts tagged with it.
So, hopefully the author will at least acknowledge he has this our request because this is a really cool plugin! I'd really like to add in my old posts that are related.
As a quick test, I decided to create another dummy-power and unless there is some other reason, the plugin will not grab the new post at all.