gitsm fetcher is not able to use a source mirror for the git submodules it pulls. Furthermore the submodule sources it pulls are not visible to the licensing infrastructure/archiver. The first part is a documented limitation while the latter isn't.
CAn you send a patch documenting this latter part please? This is one reason I have a rather strong dislike of the gitsm fetcher and in some ways wish we'd never merged it. I'd be happy to see patches making the license class error for gitsm:// urls.
We should be able to fix this via the intended implementation for Issue #11831. By reworking the submodules to use the bitbake fetcher, we'll have more visibility into their fetch process and mirrors will be used for the submodules.
patch sent to bitbake-devel.
the doc update merged in bitbake master.