Special:Nuke does not work for pages that were imported with Special:Import.
These pages simply don't get listed (in the checkbox list) when you search for
the username under which they were imported.
Here is a sample import that does not show up when you search for the username
under which it was imported (in this case, WikiSysop, which happens to be uid = 1):
http://www.mediawiki.org/xml/export-0.3.xsd" version="0.3" xml:lang="en">
<title>My new topic</title> <revision> <contributor> <username>WikiSysop</username> <id>1</id> </contributor> <text xml:space="preserve">
</text> </revision> </page>