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.