Why are these called "The All"?
Buddha's time in India there were many views about the world. This is similar to saying nowadays "The all mighty God" etc. These views are discussed in the Brahmajala Sutta. These wrong views lead you to a wrong destination. However, according to Buddhism the right view lead to Nibbana.
What Buddha saying is nobody can comprehend anything beyond six senses.

https://www.accesstoinsight.org/tipitaka/dn/dn.01.0.bodh.html