It's actually about git account switching as far as I can tell, which does make sense, you can have multiple "git" users indeed. Maybe it's the wording that is wrong? Read "account" as "user" and it might make more sense :)

  # in ~/.gitconfig
  [includeIf "gitdir:/home/user/projects/embedding-shapes/"]
    path = /home/user/.gitconfig-embedding-shapes

  # in ~/.gitconfig-embedding-shapes
  [user]
  name = embedding-shapes
  email = embedding-shapes@proton.me

  [core]
  sshCommand = ssh -i /home/user/.ssh/id_embedding-shapes

Fair point. This makes much more sense when you make it about your git identity + ssh configuration.