No it's not. Octagons have eight sides, not seven. One of the corners is a right angle.
Also, the sides aren't of even length and even the two lower diagonal sides are not symmetrical. The map does have good space management, but if he wanted to make an octagon, it makes more sense to use the walls from the Tower tileset, since they have a smooth diagonal. The regular roof is too jagged to make a shape like that work, at least for me.