It's not registered per se, and you get the same response for every single-letter dot-com besides x.com.
Considering the account was under an hour old when this was posted, and the fact that it points to the unregistered a.com domain rather than article about it, I find it more likely that it was a test/spam post. I'm really surprised it has 20 points.
Also, according to Verisign's WHOIS, these were registered on 1993-12-01, a little after x.com's 1993-04-02. This is also in line with what's written in the Wikipedia article linked by the sibling top comment. I'm not sure why IANA's WHOIS server reports it as being created in 1992.
Considering the account was under an hour old when this was posted, and the fact that it points to the unregistered a.com domain rather than article about it, I find it more likely that it was a test/spam post. I'm really surprised it has 20 points.
Also, according to Verisign's WHOIS, these were registered on 1993-12-01, a little after x.com's 1993-04-02. This is also in line with what's written in the Wikipedia article linked by the sibling top comment. I'm not sure why IANA's WHOIS server reports it as being created in 1992.