Amusing that answer. Self centered and completely egoistical.. and quite flawed. Which makes it amusing.
1. It uses flash, not silverlight. Installed base of Flash is "substantial".
2. Whine all you want about IE (I assume you mean IE6^^), visiting a site which is scripted for IE6 still works under Mozilla, Safari or Safari, if not completely well aligned.
If you think that the only difference between IE6 and a modern browser is "alignment", or if you think that the IE issues are fixed in IE7 or IE8, then you've not done any serious web development.
If your site doesn't work _at all_ on a browser without Flash, it's because your web developer is awful at his job. Period. There is no excuse for creating a site that doesn't display SOMETHING meaningful in all browsers. If it's a RIA that really needs flash to function (a game, an application with unique user interface, an application that requires camera, etc.), there should still be text explaining that to the user.
If it's anything else (navigation/page elements, video, etc.), if the person developing the site knows what they're doing, there are work arounds.
If your developer doesn't have the ability, or if you don't have the budget for such progressive enhancement, then that's not Apple's fault.