The site specifically states: “The demos below show how the latest version of Apple’s Safari web browser, new Macs, and new Apple mobile devices all support the capabilities of HTML5, CSS3, and JavaScript.”

It is a Safari Demo, not an HTML 5 demo. Thus, you need to use Safari to experience the demo. That's pretty fair.

In addition, even if they didn't do the browser check, and the demos still only worked in Safari but not in Firefox, that would be a problem with Firefox, not with Apple or Safari. They are using open standards (and you can verify this). If other browsers don't support those standards yet, that is not Apple being two-faced.