Is it possible to argue against this?
Yes, you are right. It is not an argument it is a statement. Trump said he would withdraw and that's what he is intent on doing. However, it seems that those who agree with Trump's decision to withdraw from Syria use this statement as a way to diminish any argument that doesn't support his position. It's as though I tell you I am going to punch you in the nose and next week I do just that and then bystanders tell you well you can't be surprised or shocked about it because he did what he said he was going to do. It appears that the act is separate from the announcement.