By making a blanket assumption. To me it looks like he's saying that the Home Office have to consider the other possibility and can't simply turn them away at the door no matter how much many of us may want to. It sounds less to me like an assumption than an assertion that they have to investigate the facts, and don't have the luxury of us keyboard warriors to make a snap decision to bar entry.
I think he did. You said
@Lizard was fine with risking national security.
@TenEightyOne asked how national security was affected by her being in the UK. That sounds relevant to me.