I believe you would be correct in your assumption. I asked one of the engineers how they did it and that's what they told me to much chagrin.

Seems like WhatsApp could easily choose to use a different port if they wanted to, but I guess this comes back to not wanting to have their encrypted messages get blocked.

VoIP traffic is UDP, and will always be very easy to identify. We barely have functional circumvention strategies for higher-latency traffic types, so building a circumvention strategy for traffic like realtime voice with ultra low-latency requirements is a tall order. Honestly getting VoIP to work well in the best possible network scenarios is enough of a challenge.

I assume that higher latency communication like WhatsApp's push-to-talk "voice notes" works just fine in SA though.

I can't speak for SA but a lot of folks in Oman would message each other voice messages as a compromise.

Yeah, if they push back too much the Saudis would most likely just outright ban the entire service.