Lightmapping is mostly a bunch of higher-level infrastructure that is mostly portable between rendering APIs, there is often some additional amount of work that has to be done per API but I generally expect it to be minimal.
I don't think the console manufacturers have many incentives to work too hard on supporting these shared APIs. Their business models make it much more valuable to provide higher performing and more specialized code that works better on their console. And that price generally ends up being pretty small and well worth it for a team maintaining their own engine. And that math only becomes more insignificant the more teams are switching to the main engines such as Unity and Unreal.
Of course lightmapping is just one part, but the major focus of the 4.0 release is the entirety of the Vulkan backend. There is a lot more code there than just this.
I don't think your second paragraph is quite true and it's only because Vulkan is already supported on one of the consoles (Switch) and has been promoted by them for years as a way to make it easier for PC developers to port their games. There are also things like MoltenVK (which was a prime motivator for Vulkan support in Godot) that prove that there is some demand AND that other companies are willing to pay the expense of maintaining it, so it's not something that has to cut into their business model or mess with other products either.
I'm actually not sure if Switch has a shippable Vulkan back-end. They've been saying they have Vulkan support for years, but at least 2 years ago there was no Vulkan support to speak of.
Even if it is now finally supported, most of the game developers I know of are using the NVN backend.
I don't think the console manufacturers have many incentives to work too hard on supporting these shared APIs. Their business models make it much more valuable to provide higher performing and more specialized code that works better on their console. And that price generally ends up being pretty small and well worth it for a team maintaining their own engine. And that math only becomes more insignificant the more teams are switching to the main engines such as Unity and Unreal.