So in that sense, he is correct. There's no way to ensure that clicking the merge button is indeed merging what I think it's merging. Now that I type that, we could easily just include the latest SHA in the form and check that. Problem solved!
Alternatives to this are:
* Merge manually from the command line. This is typically what we do at GitHub. * Request that people file pull requests with a SHA as the pull request head. This makes it impossible to update the pull request with future commits. * Don't use pull requests.