It's laughable because nationality isn't an automatic criteria for trustworthiness. That's why security clearances exist. For example, it's not hard to imagine

It's laughable because nationality isn't an automatic criteria for trustworthiness. That's why security clearances exist. For example, it's not hard to imagine a non-British citizen holding a clearance from a "Five Eyes" nation being far more trustworthy in that regard than a younger British citizen with a penchant for gossip. Nationality is certainly a valid criteria, it's just not very useful on its own. That said, I tend to agree with you; that line is probably best interpreted as an informal warning to set the tone.