if he secured it the darkspawn wouldn't have been a surprise. I'm assuming this is done in the morning. The darkspawn didn't just teleport to the hole when nightfall hit.
And he doesn't notice these men are missing hours later?
And if his men failed why on earth is he blaming the wardens and not you know darkspawn. Nevermind the latter makes a hell of a lot more sense.
The darkspawn more or less did teleport; it's called tunneling. They broke in from underground and swarmed in such numbers that the men were taken unawares.
The darkspawn didn't attack the tower until right when the battle was starting. There wasn't a tower full of darkspawn all day.
They also didn't come in through that hole behind Cailen's lines, they came from further down. They exited through that hole to take Cailen's force from behind while others went up into the tower.
And Loghain doesn't know what happened in the tower. He doesn't have a cellphone to talk to his men in there once the battle's started. He doesn't have a crystal ball to watch all the events.
He blames the wardens because he honestly believes that the wardens are in on some Orlesian conspiracy with Cailen. He's wrong, obviously, but it wasn't about covering up his own failure as it was shifting the focus from the darkspawn to Orlais. And I'm not defending that decision; it was a mistake, and he was mistaken about the situation by thinking it wasn't a blight. But it wasn't for the reasons you think it was for.