Just to be clear, tỷ = 10^9, it is not the "long scale" billion = 10^12?
He means "why not tỉ though?" and the answer is that tỉ is just another spelling variant of tỷ.
tỷ and tỉ are just different ways to spell it. A lot of Chinese derived (Sino-Vietnamese) terms with "i" are spelt with -y. Base syllables like duy, hy, ky, ly, my, quy, tuy (occasionally vy) can be spelt with -i instead (except for duy & tuy which are always duy & tuy and never dui & tui).