binary decision tree hyperplane parallel to axis?
Your understanding is fine. 

The hyperplane is $x=5$. 

In $3$ dimension, it is a plane that is parallel to the the $y$-$z$ plane, which can be written as $x=0$. It is parallel to the $y$-axis and $z$-axis.

You are right that it is perpendicular to the $x$-axis. The $x$-axis is the normal direction for the plane $x=5$.