I'm not Spanish either, but I think that there's a half-dozen regional languages wth official status in regional/local governments. The regional language of the capital region (Castillan) also has official status countrywide. So in practice, you could reasonably expect to get by with Castillan only when dealing with both national and regional government agencies, and with people or businesses around Spain.

But that's not what the talk show caller and host was saying. They used the "bilingualism is good for your brain" argument as a reason for websites (not sure whether they meant government, business, or personal) to have a version in Castillan in addition to whichever other regional language they use, but somehow failing to apply that same argument for Castillan-only websites, some of which would presumably be of interest to speakers of other regional languages as well.