I believe your IO make it too simply in answering no need to declare.
It actually depends on the question pose.
If ask, have you ever been charge in court of law : the answer is no
But if the question is have you ever been arrested : the answer could be YES.
But as it was let off with a police warning only, the answer could be a Yes and No... depends where and by whom the question is ask.
By the way, yours police warning is not consider a record lah
Actually it is good for you to declare if you have some discipline issues.
My friend was a gangster, who got into trouble several times with the law, before NS. The SAF MO actually took note of it and he was downgraded because of certain mental disorder.
The police kept record of everything but the SAF MO will not vertify. They can downgrade you based on medical observation and tests.