Did he? The beginning of chapter 110 seems to suggest otherwise:
Dumbledore’s grimness had returned and redoubled. “There I am, searching so hard for Voldemort’s shade, never noticing that the Defense Professor of Hogwarts is a sickly, half-dead victim possessed by a spirit far more powerful than himself. I would call it senility, if so many others had not missed it as well.”
That precise wording might carry sarcastic undertones. However, the bewilderment right before that seemed (and still seems, on second reading) genuine to me:
The grimness on Albus Dumbledore’s face lasted only an instant before giving way to bewilderment. “Quirinus? What—”
And then there was a pause.
“Well,” said Albus Dumbledore. “I do feel stupid.”
There was a long pause, then. The grey eyes searched him.
“Of course...” said Lucius slowly. “I do feel the fool now. This whole time you were just pretending to have no idea what we were talking about.”
Did he? The beginning of chapter 110 seems to suggest otherwise:
That’s sarcasm.
That precise wording might carry sarcastic undertones. However, the bewilderment right before that seemed (and still seems, on second reading) genuine to me:
It’s also an eerie echo of Lucius Malfoy: