I can't find any license information in that repository. That means all rights reserved, at least in my jurisdiction.
All plugins hosted on WordPress.org are GPLv2+ if not otherwise specified (see #2 on http://wordpress.org/plugins/about/guidelines/ ).
GPLv2 requires that a copy of the license is distributed "with every copy of the program". [0]
[0]: http://www.gnu.org/licenses/old-licenses/gpl-2.0-faq.html#Wh...
SVN repository: http://plugins.svn.wordpress.org/hyperdb/
SVN changelog: http://plugins.trac.wordpress.org/log/hyperdb/
Yes, it's not on Github (which is a shame), but that's a long way away from being closed source.