The $node_id contains the exact node name.

I suspect you have the wrong model of what's happening.

It's a bit hard to tell as you didn't show th eformat of either of your
input files, but I guess

config.xml looks like
<config>
<nodename>xxx</nodename>
</config>

if so then $node_id contains a single element node (nodename)

so

<xsl:for-each select="$node_id">

will select that node (in config_doc.xml, not any nodes in the original
document)