swupd-server v3.2.0 (and the corresponding swupd-client v3.4.0) introduce support for manifest includes. From the swupd-server release notes bundle includes: "allows bundles that "include" other bundles to depend on each other. This change reduces duplication of files in manifests and improves build times for larger bundle sets that contain many includes."
Clear Linux uses bundle-chroot-builder[1] to generate manifest includes 1. https://github.com/clearlinux/bundle-chroot-builder
Upstream swupd has changed a lot and integration with the Yocto Project would be tricky and need rework to the layer. Closing this as obsolete since any new integration would need rework and have its own issues.