This is different to a journalist saying something like this. The broadcasters presumably sign a contract when bidding for the rights to show the Premier League that involves, amongst other things, using the correct name of the stadium and club. And the obvious reason is that companies have paid huge amounts of money to have their name on the stadium. Imagine the outrage if a commentator accidently called Liverpool "the dippers."
Whether Jim Beglin said it on purpose or it was a slip of the tongue, it's equally damning. If he said it on purpose, it's completely unprofessional. If he said it by accident, it's clear evidence that he calls it that in his free time, because no-one would accidently say something that they don't say in real life. It's like those people who claim to have accidently said something racist. Yeah, I'm sure you did, but you only accidently use racist language in a situation if you regularly use it deliberately.