Arch and Gentoo are fairly popular as hobbyist distributions but they’re far less common in professional use, especially for the servers running SSH which this attack targeted. That doesn’t mean what happened is in any way okay but if this hadn’t been noticed long enough to make it into RHEL or Debian/Ubuntu stable you would be hearing about it in notifications from your bank, healthcare providers, etc. A pre-auth RCE would mean anyone who doesn’t have a tightly-restricted network and robust flow logging would struggle to say that they hadn’t been affected.