I believe that what The fox was saying was that The Juubi (its Chakra and its Evil/Emotional Intent) cannot be sensed like Naruto is able to sense the Allied Ninjas and Zetsus. However, You Can Sense It's Natural Energy if you have Sage Mode.
Originally Posted by hikonami
If you use Sage Mode to sense it, you'll see that its Strength Cannot be measured because it is Off The Charts.
I am not perfect and I defy you to prove otherwise
Growing Old Gracefully is an Oxymoron ... Mostly Moron !