I'm one of those security specialists (although not on mastodon). To be clear, if a vulnerable version of libxz were included in a distribution that we actually use; this would be an all hands on deck, drop everything until it is fixed emergency.
Having said that, for an average user, it probably doesn't matter. First, many users just don't have the vulnerable version installed. All things considered, it was found very quickly; so only rolling release distros would have it. Additionally, it appears that only .deb or .rpm based distributions would have it. Not because they are particularly vulnerable, the attack explicitly tests for it.
However, lets set all of this asside and assume a typical use is running a vulnerable system. In my assessment, the risk to them is still quite low. With most vulnerabilities, the hard part is discovering it. Once that happens, the barrier to exploiting it is relatively low, so you get a bunch of unrelated hackers trying to exploit any system they can find. This case is different; exploiting it requires the attackers private key. Even though the attack is now widely known, there is still only 1 organization capable of using it.
Further, this attack was sophisticated. I'm not going to go as far as others in saying that only a state actor could do it. However, it is hard to think of anyone other than a state actor who would do it. Maybe a group of college kids doing it for the ~~lolz~~ research? But, if the motivation us lolz, I don't see them pivoting to do anything damaging with it. And even if they wanted to, there would still only be a handful of them. In short, this is one of those cases where obscurity works. Whoever did this attack does not know or care about Joe the Linux user; and they were probably never going to risk burning it by exploiting it on a large scale.
However, setting all of that asside, suppose you were using vulnerable software, and someone with the private key is interested in your home system. First, you would need to be running OpenSSH on a remotely accessible interface. [0]. Second, you would need your firewall to allow remote SSH traffic. Third, you would need your router to have port forwarding enabled; and explicitly configured to forward traffic to your OpenSSH server [1].
If all of that happens; then yes, you would be at risk.
[0] Even though the attack itself is in the libxz library, it appears to specifically target OpenSSH.
[1] Or, the attacker would need some other mechanism to get on the same network as you.
