It's not common knowledge, he just guessed it and worded it so confidently that people assume he's right and upvote. I highly doubt that embassies of allied countries bug each other. Finding such bug is inevitable and will spark tensions between countries for no reason.

I don't know what embassies are like in the US, but I work in europe in a building with an embassy on the top floor and they have a whole other floor dedicated for soldiers to sleep/live in. Bodyguards are constantly staring at me when I go out for a smoke. One actually lightly restrained me once in the lift when I was taking out my phone.

Security in some embassies is over the top. This dude has watched too many movies and is talking out of his ass.