The link I provided explains it. They can decrypt traffic through their own devices.

From the article:

A court in Madrid heard it was assumed the message triggered alarm bells after being picked up via Gatwick’s Wi-Fi network.

Public wifi without a VPN is like sex without a condom. The connection may not be encrypted (very risky) and even if it is, you are still susceptible to man-in-the-middle attacks: https://www.garlandtechnology.com/blog/how-to-monitor-encrypted-traffic-and-keep-your-network-secure

I guarantee there will be a flood of articles about this over the next few days because of what I quoted above.

It’s also possible that one of his “friends” reported him or something like that.

In general I agree, but there’s no privacy on airport Wi-Fi. And very little at an airport in general.