In MW 1.18 action=purge requests will be split into two requests, one doing the purge and then a redirect which will lead to the actual page rendering. Therefore, in the first request where 'ArticlePurge' is called, 'ParserAfterTidy' won't be called apparently. That will happen during the second instance of the wiki where the rendering takes place. So when that happens, we can't know the purge was done. I'll look into this soon and take SMW 1.7 under consideration as well.
Topic on Extension talk:SemanticUpdateOnPurge
I really love this extension as I sometimes rely on ExternalData as values for SMW properties. I was wondering if there was any update with fixes for MW 1.18+ and/or SMW 1.7+.
Not that easy to make it compatible again but I got some ideas, so If I find the time I will try getting it to work. Hasn't been a high priority to me though since I am pretty busy with other projects currently. Would be nice though having this feature again on some of my Wikis so I will give it another try.