I don't think that's true.
Are you referring to this in the manga?
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I think he meant the radius of his attack. We know how big a blast radius can be caused from Dust release attacks. For example here:
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But this
doesn't mean that his Jinton can't reach the ground. Look at this attack:
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The two situations are different. In his fight against Deidara, he just didn't want the
radius of his blast to reach the turtle, it had nothing to do with the fact that his attack can't reach the turtle. I reckon if Onoki used an attack like the one he did on Madara as shown above, I think he could hit the turtle.
So I think Onoki can still hit Minato on the ground.