Kernel development is well-funded, both via grants and by direct employment at big tech companies, and if they wanted to properly triage and annotate vulnerabilities, and provide reasonable assessments of what is or isn't likely to be a security risk, they absolutely could. They almost certainly could go to Google and say "we need two people full-time on your payroll for this" and they would get it.
I don't want to dunk on them too much because they're generally doing God's work, but these absolutist security stances are not worth being taken seriously.
It's basically saying that they can't possibly provide a valuable service for 99.999% of the install base because there might a hypothetical person out there using Linux in a really weird way. If Microsoft tried to make an argument like that, they'd get crucified.