I found a workaround to this from the following source, the script that gets only the messages and not threads is:

function listMessages () {
  // Only return messages matching the specified query.
  var msgs = Gmail.Users.Messages.list('me', {'q':'label:InProcess'}).messages;

  msgs.forEach(function (e){
   var message = GmailApp.getMessageById(e.id); // Fetch message by its ID
   var plainBody = message.getPlainBody();
  });

}