It might mean that Naruto never found the ultimate answer to peace. It was just based on cooperation, and that seemed to be working until Character Z arrived. In my opinion it all depends on how it is written, I have very little information on the topic, so I can't really accurately judge it yet.
People complained about Naruto establishing peace.
People complained about Naruto's way of establishing peace was inferior to Sasuke's
People argued that Naruto's peace wouldn't last forever.
What Naruto established was peace with the five villages together, but what he/they couldn't stop is the conflict between outside sources who don't follow the Ninja system.
Technically, it wasn't even Naruto who established the system of cooperation between the five nations. Oonoki, Ay, Mei, Tsunade and Gaara did it, to fight together against Obito, Madara and the Juubi. Naruto was their inspiration to keep this system up after the war.
And yep, there are still outside-sources that trigger violent conflicts, children are still trained to be killers etc. That can't be "peace" in my book. Naruto isn't special, his way is just one of many and he will be added to the list of failed peace-bringers, like Hagoromo and Hashirama.
Well he never said he found the answer, he said he wouldn't stop until he did. What he did confirm was that he had a plan, or an idea for it. Which involved everyone's help.
Sure, he didn't say it. But the manga implies that he found it.