In 1984, Winston interprets O'Brien's indirect reference to Syme as a shared act of thoughtcrime. How is this later shown to be an example of irony?

Syme is one
of the more interesting minor characters in the novel.likes to talk to him because Syme is
working on refining Newspeak and can converse intelligently about it. In fact, Winston realizes,
Syme's insightfulness will be his own undoing. As Syme chats to Winston over lunch at work, it
strikes Winston that:

One of these days, thought Winston
with sudden deep conviction, Syme will be vaporized. He is too intelligent. He sees too clearly
and speaks too plainly. The Party does not like such people.

Not long after the lunch conversation, Syme disappears, as Winston had
predicted:

Syme had ceased to exist: he had never
existed

Whenstops Winston in the hall to talk him and ask
him to come to his apartment to get an advance copy of the latest Newspeak dictionary, Winston
is certain O'Brien is referring to Syme when he mentions a "friend:"

"But you write it very elegantly," said OBrien.
"That is not only my own opinion. I was talking recently to a friend of yours who
is...