signal requires a phone number to sign up. a phone number could be used to trace your signal account back to you. so why do people, especially privacy enthusiasts and experts (like edward snowden), still use it and endorse it when it lacks anonymity in that sense? i get that people could use a voip number or something to sign up, but still.
I wouldn't call it a mistake, more like being caught between a rock and a hard place, where Android basically forced them to give up on SMS support even though they'd have liked to keep it: https://community.signalusers.org/t/signal-blog-removing-sms-support-from-signal-android-very-soon/47954/57
But yes, it was really nice when I could use it as my SMS app. Then again, very few people in my country use SMS in the first place - it's all WhatsApp, and it was never able to have support for that. Luckily, most of my friends have adopted Signal by now.