Good points. Though I'm a bit confused by your reply. Are you saying that LineageOS folks do not always or, at least, mostly use the latest AOSP common kernels for their relevant ROMs (as opposed to "some open source kernel fixes")?
I don’t know. I’m saying that custom rom use kernels that make your phone work. In the best case that involves shipping 1) the driver and firmware blobs the vendor provided while supporting the phone and 2) a kernel that is binary compatible with those blobs. Because of how Linux works, in the best case (2) is an old kernel of the same major version as the vendor shipped with the phone, with maybe some security fixes that made it into the mainline kernel or in the Android kernel. But if your stock rom has security bugs in e.g. the wifi driver, graphics driver of baseband firmware, your custom rom has those exact same bugs. Even if the custom rom is years newer than the latest vendor update.
Some diligent LineageOS projects are known to incorporate some open source kernel fixes sometimes, or grab newer blobs from other phones from other devices. But there’s only so much to they can do. In general, it’s true to say that older devices with community Android support are not completely up to date - the kernels are old, and vendor drivers are not getting updated. Outside of making big usability concessions in projects like Replicant, the community can’t do much here.