The README refers to github releases for downloading released sources, but that only allows downloading an automatically generated git snapshot.
The generated source bundle is different (includes md5sums, maybe other differences too). This is uploaded to S3 for download through openttd.org (I think), but perhaps it should be uploaded as a release asset to github too?
See OpenTTD/nml#111 and OpenTTD/nml#113 for this same issue and the fix for nml.