If you agree that the required ability is "use my portable cell-connected computing device to exchange messages with others on their similar devices", then absolutely, I agree that there's a duopoly there (I'm ignoring the possibility to use alternative app stores on Android, because it doesn't seem particularly central).
But if you instead think the required ability is "be able to exchange messages with others in more or less realtime", then the duopoly in the portable cell-connected computing device world is of much less importance.
For years, because I don't have a cell phone, I was excluded from communications taking place via SMS (there were a few 1-way linux-accessible gateways, but they didn't really change anything there). But I didn't feel that my ability to communicate with others was really particularly impaired, and eventually Telegram (and others came along) that I could use from a desktop, and I was more or less back at parity for "real time communication with others".
So whether or not the android/ios duopoly matters in this sort of instance depends on precisely what functionality it is that you think people have a "right" to ...