The default depth for Xvfb was changed from 8 to 12 on 2005-03-23 by Roland Mainz and its manual page was changed accordingly.
The code change was reverted/disabled on 2005-07-23 by Stuart R. Anderson but the manual page was not reverted.
Sorry about the phenomenal bug spam, guys. Adding xorg-team@ to the QA contact so bugs don't get lost in future.
I've just pushed a fix to git. Thanks for your report!