They haven’t denied it. The quote I heard was that “they have no reason to believe” it is a terrorist act. That’s not a denial.
In fact, it’s nothing, really. They’re unwilling to say anything until they know something. Why is that bad?
And let's say for the sake of argument that they knew it was terrorism, and they had an idea where the terrorists were. By not saying right away it's terrorism, it could lull the terrorists into a false sense of security and make them easier to catch.