The equipotential lines (surfaces) are perpendicular to the field lines, which would mean in they case that the same line that is perpendicular at point a would have to be perpendicular at point b. While this is not drawn in, a mirror through the straight field lines (passing through the two charges) would reflect point a into point b. If you don't see this, that;s fine, but if you do, then it should help in believing that point a and point b have the same electrical potential.
Given that it would take the same amount of work to move a test charge from point a to the charge on the right as it would to move a test charge from point b to the charge on the right, AND the same amount kinetic energy would be generated if a test charged moved from point a to the charge on the left as would be generated if a test charged moved from point b to the charge on the left, then a test charge would have the same potential energy at point a as it has at point b.
You are correct on all accounts.
