If the two surfaces are planed and parallel to each other, making an
edge perpendicular to one face also makes it perpendicular to the
other...

Simply jointing the edge will not change the relationshiop of the to
faces to each other...if one corner angle is less than 90, the other
face/corner angle will be the complementary angle.

Ergo, if one buys surfaced material of the desired thickness, one can
more easily do w/o the planer (thicknesser) than the jointer.