This does strike me as kind of weird but in the same way it's weird that github lets you author commits with other people's email address and it shows up with a reference to their github account. A lot of github has very strange UI edge cases which come from the fact their features are a leaky abstraction over core git operations. Since the PR diff shows no changes were made and the merged commit is one that's authored by an actual contributor, I'm not sure it's as much a security vulnerability as a curiosity.
Git and Github both allow you to put whatever email you want. If you care about being certain who is committing to your repo, you should ignore the email and only look at the commit signature.
If you're talking about the issue OP is discussing, it should still be possible even if it's a signed commit. 61f3741 is a signed commit in the linked PR.
This just re-uses existing commits on the repository. The commits can be signed and github will still show "merged by X" if neither X nor the author of the signed commit merged the PR.
So really it's "if you care about being certain who is committing to your repo, you should ignore who github says is committing to your repo", which, to my earlier point, is technically understandable when you dig into it but nonetheless a little weird from a UX perspective.
If you're talking about forging the commit author, that's also weird. It makes sense in the decentralized context of git, but not in how most people use github. Nobody is saying that it isn't allowed, but the fact that github allows it is really an artifact of the fact that git allows it. In the github web app, your account is email verified, so it's weird that someone can generate commits which (in the UI) link to your email verified github account that were not actually created by you. Most people don't expect webapps to work this way, even if git might. It'd be similarly weird if facebook allowed people to create posts on your behalf and we told users "oh that's not weird, you should really verify the GPG key of your posts".
