I know it's dicey regarding Spock lying. Vulcans don't do it often, but they can.
He's omitted the truth, exaggerated the truth, but flat out lied like here and not for any real reason?
In several other eps. he lied - The Enterprise Incident, The Menagerie & movies: Star Trek II: The Wrath of Khan, Star Trek VI: The Undiscovered Country but those could be seen (I guess) as for the good of the ship/crew. Why the lie in this ep?
Vucans lie ALL THE TIME. The whole "Vulcans don't lie" was started when he had the exchange with the Romulan Commander of:
Romulan Commander: "...there's and old saying; or perhaps it's a myth, that Vulcans cannot lie?"
Mr. Spock: "It is no myth, Commander..."
The problem is: Spock was LYING to the Romulan Commander when he stated that; and that's clearly evident if you watch the entire episode in context front to back.
It was established that Spock could/would lie in the first season two part episode that ree-cut the original pilot footage to make it usable for the series - "The Menagerie".
Then in "Journey To Babel" we had evidence full blooded Vulcans had no problem lying as Spock's father lied to both Captain Kirk and his own wife in an attempt to conceal his bad heart condition.
People who site Spock's line from "The Enterprise Incident" as 'canon proof' that Vulcans don't lie are in fact, taking the line out of context when you view the events of that episode in their entirety. If anything that episode proves conclusively that Vulcans can (and will) lie if they see a logical reason to do so.