I don't understand why the author thinks 150km/s is somehow powerful enough?

Fastest baseball players can throw that fast, and while the ball can be pretty bad if hit its a person, it won't leave much of dent in a tree even if it was a rock and not a ball...
Blunt objects don't penetrate a target's body as well as a bullet does, but Hiro's wind bullets are literally tiny cones, their point ends could pierce through just as well, if not better than a rifle bullet with less speed needed to do so. It comes down to the surface area where all that force is being focused at the point of the bullet/cone, whereas with a baseball, the force is being distributed evenly throughout the affected area, not causing enough damage to penetrate/pierce the target on impact. A needle doesn't need to be shot from a gun to penetrate skin for the same reason, while a baseball would probably need to be faster than the speed of sound to have a chance to penetrate a target,
if it doesn't explode from the collision on impact.
Physics isn't my specialty, but I'm
pretty sure that's the logic behind this.