He's not restricting GitHub from hosting it, he's just using an artifact of GitHub's implementation (or, well, a hypothetical GitHub implementation - I'm not sure GitHub actually blocks submodules that reference the relevant commit hash?) that happens to make GitHub refuse to host it.
GitHub is free to change their minds about the implementation and start accepting those commits again. That is, you, as a recipient of the code, are free to ask GitHub to host it, and to offer them a license under the terms of the GPL; GitHub is free to accept or to decline, same as if you sent me the code, I am free to host it on my personal website but I'm also free to not host it.
(It would feel more like violating the spirit if it, say, prevented you from uploading it to a GitLab instance you self-host... but since you self-host it, you could just patch out any suppression code, so that case can't come up.)
GitHub is free to change their minds about the implementation and start accepting those commits again. That is, you, as a recipient of the code, are free to ask GitHub to host it, and to offer them a license under the terms of the GPL; GitHub is free to accept or to decline, same as if you sent me the code, I am free to host it on my personal website but I'm also free to not host it.
(It would feel more like violating the spirit if it, say, prevented you from uploading it to a GitLab instance you self-host... but since you self-host it, you could just patch out any suppression code, so that case can't come up.)