I have no idea if she told all she knows or not. What we do know is that the police who were actively searching for her missing stepson weren't done asking her questions when she clammed up. LE was still in the very early stages of an active investigation to find a missing child - her own stepson - when Terri Horman stopped cooperating. LE was still amassing new witnesses and new testimony from others at the school and on the roads the day Kyron vanished - in order to find Kyron at a stage when (if alive) he could still be saved, and (if deceased) his body had a better chance of being found - when Terri Horman chose to stop cooperating. She knew she was the most important witness, being the last family member to have seen him at the most crucial time of the day he vanished, and she backed out of the investigation, no more help for the people trying to find/save her stepson, no more searching for a missing child, no more contribution to the investigation.
How likely is it that the innocent parent of a missing, possibly kidnapped, possibly murdered little kid decide they were all done looking for him within weeks because after a few conversations with police they couldn't think of anything more they could contribute and so they were done participating in the search?
I imagine the reason she stopped cooperating was because it had become obvious that LE were busy painting a bulls eye on her back. Under those circumstances anyone who continued talking to them would basically be tying their own noose, you would have to be pretty stupid to do that.
And yes, if you were in that situation you would stop talking as well. Being innocent is not enough, once LE have decided that you are "it" you are in serious trouble, your innocence will not protect you.