What does basipetal succession (of flowers/leaves) mean?
The definition you quote is a bit misleading — what matters is where the lateral flowers are connected to the inflorescence axis, not where they are physically in space. 

Thus, due to the way these cymes develop (starts with a terminal flower and then a lateral bud grows out to form anther terminal flower, etc.) they must be basipetal because the first flower to develop is the terminal flower.  In other words, for cymes the lateral buds tend to overgrow so it looks like they are above, but what really counts is where they are placed on the inflorescence axis (which is below).

You may find this explanation clearer than the one in your textbook.