Even if you factor in the environmental score? I realize it's a lot more work, but it basically allows you to tune the score to get any value you want.
If you're an online service where untrusted users can submit arbitrary YAML, and an out-of-memory exception is a severe problem, then it's severity 10.
If you're an online service that doesn't use yaml in any way, but your web framework bundled the library as a transitive dependency because yaml is one of their five supported configuration options, then it's severity 2.
The problem is figuring out which of those situations you're in takes a load of time - and the flow of CVEs is endless, as CVE numbers are given out like candy at halloween. Often it's quicker to just update to the latest version of the YAML library.
And then you can have all kind of other fun things, like users don't upload YAML so you think you're safe, but a clever hacker figures out another minor parser error that makes your JSON seem like YAML and suddenly YAML.so has loaded and you're now parsing a document that shouldn't be possible.