But he doesn't know everything about it. For some issues in his field, he's mistaken.
Porter hinted that Burke was an anti-capitalist, as opposed to Adam Smith. But Porter doesn't know what he's talking about. A story is (it's hard to get really solid info about stuff so old) that Adam Smith said Burke was the one man who thought the same way as him on economic matters. Porter further implied that Burke was an anti-liberal, anti-Enlightenment man, but that's ridiculous: Burke was a lifelong liberal reformer and member of the liberal political party (Whigs).
Porter has Godwin wrong too. He thinks Godwin a hypocrite for his views on marriage and his actions, but he simply does not address Godwin's reasoning. He glosses over the issue quickly, and hopes I'll take his word for it, but I'm not going to, I have an informed view on the matter and Porter offered nothing to change my mind. Porter also makes a factual claim -- with no citation -- about Godwin wanting his daughter and Shelley to marry which I've seen nowhere else (I've read many books specifically on this topic).
Porter is certainly not the only person to get Burke and Godwin wrong. Thomas Sowell has them atrociously wrong, even misquoting them.
These are examples of the simple fact that no expert knows everything, and especially in broad fields there is so much to know that very few people study all of it.
People have holes in their knowledge, even experts.
In my experience with experts, when they speak to an area that I've studied a lot, the majority of the time they get it wrong. They are not usually right, but usually wrong.
Most philosophers have epistemology wrong. It's not just that they disagree with Popper but that they don't understand the issues and address his best arguments. They're outclassed and ignorant.
Most historians of the relevant types have Burke wrong. And most have Godwin wrong.
Popper says most historians of Plato and Aristotle are badly mistaken (and argued his case). He persuaded me. Popper isn't the only one to say this. Godwin had a negative view of them both. Bronowski recognized that Plato's politics were bad. And of course there were Greeks who didn't see eye to eye with Plato or Aristotle (such as, respectively, Pericles and Xenophanes).
These are cherry picked examples. I've simply taken the areas where I know a lot and focussed only on them. Are these same experts right about other stuff where I don't know what I'm talking about? Maybe they are and maybe they aren't, beats me.
Some may reply with arguments that I'm wrong on the examples I've given. So what? First of all for these narrow areas, I'd say I'm an expert. But if you want to think for yourself instead of accepting my expert knowledge, go right ahead. Second, I'd suggest we settle it the way everything should be settled: not by appeal to authority, or consideration of the sources of claims, but simply by critical debate. Post in comments and I'll tell you why you're wrong or I'll concede, simple as that.
Anyway, here's what I think, the lesson to be learned: when I'm in a serious position to judge an expert opinion for myself, I find it lacking the majority of the time. Why should the experts be more reliable for issues I haven't personally studied? I think the experts are simply unreliable all the time, and make mistakes constantly. That makes it all the more important to think for myself, and study important issues for myself.