9 has been done many times in human history too, for some reasonable definition of "create a better artificial optimizer."
Anyhow, to answer your question, I'm just guessing, based on calling "difficulty" something like marginal resources per rate of success. If you gave me 50 million dollars and said "make 2 happen," versus if you gave me 50 million dollars and said "make 9 happen," basically. Sure, someone is more likely to do 2 in the next few years than 9, ceteris paribus. But a lot more resources are on 2 (though there's a bit of a problem with this metric since 9 scales worse with resources than 2).
That's why 9 specifies "recursively self-improving", not "build a better optimizer", or even recursively improving optimizer. The computer counts for recursively improving, imho, it just needs some help, so it's not self-improving.
From Hacker News.
This made me laugh, but from the look of it, I'd say there is little work to do to make it serious. Personally, I'd try to shorten it so it is punchier and more memorable.