It proves it? How?

This is more like an exception than the norm and Berkshire is not really something to generalize.

Most investment funds of that size do chose to maintain proper websites.

This is more like Berkshire branding, look at Warren living in the same house despite having billions (while conviniently ignoring dude owns personal jets), look how we have such a grandpa website, look at our humble branding, we are not like the other greedy bankers and are perfectly moral agents you can "trust" etc.