I used to work at WhatsApp (until the end of 2019) on many things, including special pricing (aka zero rating); we did not work with airlines, and would not have participated in a project where messages and attachments where treated differently.
That said, technically there's two pretty easy ways to do it for WhatsApp traffic, and then there's the way I suspect they're doing it...
a) chat runs on different ips than attachments; always has, most likely always will (other than some transitional HAProxy at the old hosting when nearly everything had been moved to the new hosting).
b) WA chat is not HTTPS (or even TLS) and attachments are. Chat also cycles between different ports, so you could just block port 443 and be good.
c) I actually suspect, based on poking around a little that it's mostly just killing connections that use a lot of data. Maybe in combination with some other things. Being on a plane doesn't really put me in a debug the network kind of mood, so I never got to the bottom of it, but I'd regularly be able to make short connections to my home network while on the messaging plan, at least when this stuff was new. OTOH, I think I recall being able to connect through the WA VPN while on a plane on the messaging plan, but that was when we had a publicly available, but not publicly linked list of IP addresses on our website; I have no doubt that DPI vendors had that list.