I've received messages that way before while on the wifi (including "Welcome to xyz! Your flight arrives at gate B99 and your connection departs from W77. It's about a 20 minute walk; map out your connection here..."). But it's a mixed bag when sending. Maybe my phone is trying to use SMS to send and sees I'm otherwise in airplane mode.

You have to have "RCS Chats" enabled in Google Message settings at a minimum, but I do and still have issues with in flight messaging.

ETA: Google says RCS Chats requires both participants have it enabled, so if you're trying to message someone without it on, it won't work over wifi. Perhaps that's the problem?