I wouldn't say "horribly" inaccessible, but it needs some work.
As Olga said, validators are useful but not sufficient to guarantee the accessibility of one page. The "Cynthia says" report leaves blank for the checkpoints that need to be manually verified.
I'm no accessibility expert but a review of the page you mentionned shows a few issues (e.g.: tree doesn't show up when js is disabled, h1 used for the "registered users" box, use of blockquote to indent text, not enough difference in color for links - but that one probably comes from theme-zen 2.4.0, it has been fixed in 2.4.1-, keyboard only users and screenreaders would have to go through all the tree links before getting to the main content).
Anyhow, this is an example of xowiki displaying content, accessibility of the pages to insert and manage content would have to be reviewed too.
Having an accessible wiki would be a great contribution for the community. For this reason, although I have to deal with other commitments, I'm willing to help to address accessibility in xowiki. We can talk about it in Valencia if you'd like.