Randal,
Yes I'm sure and the response from IBM is correct.
Regarding the installation of the Extension Library you have 2 options:
1. Download the latest version from extlib.openntf.org and install that.
2. Upgrade your server using a (by IBM provided) update/ installation file to version 8.5.3 Upgrade Pack 1. After the upgrade your Domino server version has changed and the extension library is available.
Upgrade pack 1 is an addition to the Domino 8.5.3 containing a release of the extension library that is officially supported by IBM. The version you download/ install from the OpenNTF site isn't supported by IBM.
BTW: I just re-checked the downloaded extension library zip file from OpenNTF and the filename isn't updatesite.nsf anymore, it is updateSiteOpenNTF.zip.
Mark