I'm not an intelligence community expert, so I can only guess what he meant by that sentence--but my guess would be that there are countries that put a lot of money and resources into counterintelligence, of which the UK is unsurprisingly one (and I'd imagine Israel too, and likely Canada, certainly the U.S.), while countries like Austria don't have very many counterintelligence resources and are therefore easier to penetrate with more "ordinary" spies.

-J