Yes, this is correct.
I could not have used SMS for most of the day since I worked on the top floor of a multi-level basement. I had wifi and a VPN though.
At the time I moved to signal, Google Message, or chat or whatever was being replaced with Allo and Duo and everyone was pretty un-jazzed about the whole thing.
Yeah I'm not super happy with Signal's level of openness, but I also wouldn't call it proprietary. It is open source and I could run my own server if I really wanted to, but that would just make it harder.
At the time we were moving from a walled garden to one of 3 encrypted OSS solutions, and I had all of them until we naturally coalesced on one.
SMS at the time didn't work due to not having cell service, and while we "have" RCS now to solve that issue, I don't know anyone who uses it.