The message was sent privately. After reading about Snapchat security at https://courses.csail.mit.edu/6.857/2016/files/11.pdf, it's clear that even in the absence of E2E encryption, all message are sent to the Snapchat API using TLS and even if using a public airport Wifi, this message should not have been easy to intercept.
This leaves only a few options about what is happening here:
Option 1: Big Intel has totally broken TLS and can see all TLS traffic in the clear in real-time
Likelihood: very unlikely, I don't think this would stay secret for long.
Option 2: Big Intel got itself a certificate for the Snapchat domains and use it to MITM Snapchat traffic.
Likelihood: very possible but unlikely to be the source as I don't think they would use it too openly on random users as it's too easy to get noticed if used widely.
Option 3: Snapchat is actually monitoring all messages and reported the message to authorities themselves. Alternatively, Big Intel is in bed with Snapchat and all message get processed by an intelligence system.
Likelihood: I think this is the only explanation that makes sense.
Since it would not be very good PR for Snapchat to admit that they are monitoring all messages, the authorities must have invented that airport wifi monitoring story. Who even use an airport wifi in their home country anyway... Pretty sure he had 4G and no need to use the crappy airport wifi.
Anyway, that's the only way I can make sense of that story.