First of all this plugin is VERY old and not updated for many years. I’m not a developer, but i think logic on this page is to block every listed client if none is selected (and what’s the point to block all the clients anyway?). As your custom Smack API client is not on the list, it is not blocking it. But it blocks Pidgin, Exodus and older Spark versions. It probably identifies Spark by its Smack version and as Smack library was updated in 2.8.3, it can’t identify it and is not blocking it. This blocking is very weak though, as far as i know it checks for some string in clients connection, but this can be altered and this plugin will be fooled.
To see if plugin works try with Pidgin.