A pull request, literally, need only be moving code from one repo to another.
The signaling mechanism etc. are not and shouldn't be specified by git. How code moves around should be dictated by your organism, copyright, access levels, not some guy's interpretation of what "pull request" means.
That means, yes in some instances there is no 'master' version of the software, there are multiple flavors with different release characteristics.
E-mail I suppose is an adequate communication medium, if people want a specific review mechanism and triggers adding one to source hut (assuming FOSS) shouldn't be hard, perhaps the authors intention is to make it difficult to regress to a centralized model.