He's right. You'd want to arrange them in a square for comparable coverage so it'd be sqrt(10) = ~3.2.

It feels somehow nice and correct that an effort to reduce a 3d problem (basically a lot of our boom goes up or down, which is not necessary in most cases) to a 2d one changes it from a gain of x^(1/3) to x^(1/2).

The OP presumably meant a line of blasts though? That would possibly be the least efficient orientation after blasting the same spot 10x.