You are absolutely correct. I noticed this a long time ago and guess I forgot to mention it here. nxTools contains a companion dll that enhances TCL with a few extra features and somewhere along the line upgrading the TCL library to newer releases broke parts of the dll.
There are no trivial solutions immediately available, however v8 of ioFTPD will fix this problem in 2 ways. The information the dll provided that is currently broken will now be available via iTCL (i.e. an ioFTPD extension to TCL) so nxTool's dll won't be necessary and we can fix nxTools by replacing the dll call with the ioFTPD call. That should be relatively straightforward to do. In fact is someone else wants to fix the dll right now it probably wouldn't be hard either.
I have also written most of a replacement for this command as a standalone script which is why ioFTPD v8 will have this info
I think my solution should be superior because it will use section, mountfile, etc directly from ioFTPD instead of trying to do it in the script by parsing files. It's also much smarter. nxTools pre-dates those new features though so it did the best it could at the time, but depending on parsing ioFTPD files and requiring updates to it's own config file is something I'm trying to avoid in my stuff.
Short answer: disable the command by undefining it and wait a bit for the replacement.