That's still only one source code for all of them. (Nougat never had treble) the difference there between actually one single image and five is build time. There is no additional development needed.

And that still means that with five images you target several hundreds (probably thousands) devices. More than lineage will ever support

GSIs should be closer to the goal - Lineage and other free software people should really push manufacturers to publish their GSI images to make Lineage porting easier.