The older version of that page explains it better: https://web.archive.org/web/20200220182425/https://support.signal.org/hc/en-us/articles/360007321171-Can-I-se

The older version of that page explains it better: https://web.archive.org/web/20200220182425/https://support.signal.org/hc/en-us/articles/360007321171-Can-I-send-SMS-MMS-with-Signal- https://web.archive.org/web/20200220182425/https://support.s... "Signal Android can be configured as your default SMS/MMS app" "Apple does not allow other apps to replace the default SMS/messaging app"