"Apple replied asking not to publish the details as they plan to address the issue in the Spring 2021 security update"

Besides the fact that I'm always torn if vulnerabilities should be published if the vendor has committed to fixing them: I don't quite understand why a fix can't make it into the next big release that's around the corner (macOS Big Sur and iOS 14). Do large vendors really have such long development pipelines that changing anything is impossible for months? Or do they just have thousands of similarly serious issues and need to prioritize?