There are lots of possible ways to identify WhatsApp traffic, but I never had a chance to figure out what they were really doing. During that time period, if I was on a flight, I was usually with my young child and it's hard to keep focus for debugging networking on a plane, anyway. What I saw when I was looking was more like what joshvm describes elsewhere. The messaging only plans seem to allow most low bandwidth connections, with high latency, but they'll actively supress some things, and others stall beyond some threshold; sometimes you could get a couple media files to transfer, but then it would stop, etc. WhatsApp was engineered to work with the world's terrible networks, so it will usually work ok for messaging as long as packets get through eventually; connection and ping timeouts are long on client and server, because sometimes it takes a lot of seconds. If DNS doesn't work, that's fine too. Multimedia would usually retry and resume enough to work even if connections didn't last long, so I'd guess there was something actively supressing that, but I don't really know.
FWIW, chat isn't TLS, so SNI isn't the answer there, although at least in the past, the protocol was very identifyable. Been gone for a while and don't regularly tcpdump my connections to WA anymore, so I don't know if that changed though. Multimedia is https, and probably has SNI, although that used to vary by platform.