Severus didn't read Al's fears, like reading words written on a page or even written on the air in lines of fire. He felt them, with all the immediacy of his own fears.
So he offered those fears to Al in turn: his own need to be protected, which he judged to be greater than Al's, because he knew perfectly well that the full brunt of public displeasure would fall on him, not on Al. Al would at worst be pitied as his victim. It was he, Severus, who would be vilified as a sleazy predator on innocents, an abusive betrayer of undeserved trust. It was he who stood to lose the Headmastery, who might possibly fall from the heights of wizarding society to the dregs.
It had been true, what he'd written, that he wanted to protect Al from their lies. But it was only part of the truth, the easiest part to admit to.
Only here, where no-one else could possibly know but Al, could Severus bring himself to admit his own weakness, and how bitterly it shamed him.