This is the part I don't understand. Who is immunizing? His critics? Or his work is immunized by having gone to public school?
Seems to me that what he chose to do as an adult should not be discounted because of where he went to school as a child. From the article, and what else I know of Banksy, it sounds like he is a very self-made person. As you pointed out, it also sounds like his early education gave him the skills to see the absurdity and hypocrisy in the world and address it creatively. Why is that a bad thing?
I'm confused.
I'm suggesting that people who have an English public school type education are less likely to engage in organised political action that is oppositional to the current arrangements than those who have not had that education. The immunisation comes from the presentation of Banksy as a lone and mysterious eccentric by publications such as the Smithsonian Magazine. That would be harder if there was an organised Banksy posse with a programme who ran summer schools &c
In the UK, the 'public' schools are actually private and the more famous ones charge a lot of money. They are also registered charities which annoys some other people trying to run real charities (the ones that help poor people), but that means there are a few places made available on a scholarship basis at little or no cost.
A good public school has small classes (8 to 12), individual attention, and focuses on the development of what they call 'character' in the pupils. The result is an education that provides considerable self-confidence (sometimes misplaced by the way) and self-reliance.
When you think about it, an ideal education for an individualist social critic. Whose career can then be presented as 'eccentric' and thus the threat immunised.