I know people who've met Clarkson socially. They thought they'd hate him but came away saying he was actually a tidy bloke. He undoubtedly plays up for tv, media etc but it's all a mickey take that's not meant to be taken seriously - not that he's got much time for those who do take it seriously. Also very happy to have the mickey taken out of himself.
I suspect (especially given the 'wordplay' argument) that they judged this one wrong thinking it was akin to 'Jocks' for Scottish people or 'Taffs' for Welsh people, or 'Frogs' for the French - which they'd have no hesitation in using.
He's always ripping into the Welsh. I'm Welsh and I find it funny (see his comment quoted above - which is very tame compared to other stuff he's said - including putting a map of Wales to cook in the microwave because it was a waste of space). It's just banter.
Never seen him stray into 'serious' racist territory (as some have touched on 'What if he'd used the 'n' word?' etc) so, as I say, I expect this was a misjudgement - especially as, despite others finding it 'hard to believe', this is just not a term in common use in the UK. I had never heard it before this thread.