It is not a given that a security hole in big sur also exists in mojave. The newer OS’s contain support for newer devices and use different and newer libs.I don't think that's accurate. Apple last issued a security update for Mojave on July 21st. Since then, Apple has patched security holes in Catalina and Big Sur (just last week, on the 13th) but not in Mojave.
If you're still running Mojave, you have active/open/known security holes that unfortunately have not and almost certainly will not be addressed. For better and worse, to maintain a secure system, you have to upgrade to at least 10.15.
what you state is not impossible, but it’s far from a given. And mostly the people who investigate such holes (namely: those who would abuse them and those who report them) would quickly discover such mistakes.
it is not also uncommon to fix in newer versions, but merely mitigate in older ones, which they could do through gatekeeper.