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):
<mediawiki xmlns="http://www.mediawiki.org/xml/export-0.3/"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.mediawiki.org/xml/export-0.3/
http://www.mediawiki.org/xml/export-0.3.xsd" version="0.3" xml:lang="en">
<page>
<title>My new topic</title>
<revision>
<contributor>
<username>WikiSysop</username>
<id>1</id>
</contributor>
<text xml:space="preserve">
{{stub}}
[[Category:MyCat]]
</text>
</revision>
</page>
</mediawiki>