The prevailing wage for H1B software engineers is a very good salary for most software engineers in the world.
It's common for the prevailing wage to be double what that person could earn in their country of origin.
And still exert downward pressure on wages. Just by virtue of having more people in the pool.
That's the parent's argument. Prevailing wage in country A is X. Prevailing wage in country B is Y, and according to your numbers X > 2Y. Person from country B emigrates to country A and accepts wage 1.9Y, putting downward pressure on the jobs in country A at wage X.