According to this site, Red Hat Enterprise Linux is based on Fedora. Is this true?
It all started with Red Hat that was the base start and the first widely-available beta release of Red Hat Linux was on october 31 1994 with version number RHL 0.9. On september 25 2003 they started the Fedora project as the developer distro (you are right, also for RHL). So Red Hat became the enterprise distro where Fedora is still real open-source and top of linux development.
Yes, RHEL is based on Fedora. See this Fedora Wiki article: http://fedoraproject.org/wiki/Red_Hat_Enterprise_Linux
Red Hat Enterprise Linux (or RHEL) is a commercially supported derivative of Fedora tailored to meet the requirements of enterprise customers. It is a commercial product from Red Hat which also sponsors Fedora as a community project. Fedora is upstream for Red Hat Enterprise Linux but there are several other Derived distributions available too.