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?
I've never thought that Vulcans couldn't lie, and that Spock was definitely capable of lying through his teeth if he wanted to. However I never understood why Saavik was so surprised at Spock for "lying".
By the book! Regulation Forty-six-A:
'If transmissions are being monitored
...no uncoded messages on an open
Saavik steps out in front of Spock.
Hours instead of days, Saavik.
As far as I can tell, Spock neither lied nor exaggerated. He used a simple word substitution cypher, per regulations.