No, it doesn't mean that either (extermination of males). It just means that only Steppe Y-chromosomes survived. This could be the result of extermination, or it could be the result of stochastic drift, or selection, or any number of other scenarios that result in the loss of some set of Y chromosomes.

Realistically, extermination of males is exactly what it means. It cannot be stochastic drift; the pool of chromosomes dying out is too large. It can be selection (it is selection), but it can only be the kind of selection that involves every native male failing to have children at all, because the Y chromosome does not recombine.

First of all, given that we only have a limited sample of the Y-chromosome space, I don't think we can conclude that no native Y-chromosomes actually survived. That aside, it's easy to get selection for specific Y chromosomes without extermination, for example if you have a ruling caste whose males are favored (e.g. by primogeniture) or through preferential mating by lineage. See: the prevalence of the (presumed) Chingisid chromosome here: https://doi.org/10.1086%2F367774

> were exterminated

or expelled, or otherwise prevented from being allowed to breed.

Such as castrated.

From what I've read, that's what the Mongols did. It's also what happened for many indigenous groups in eastern North America. Except that the males were escaped African-American slaves. I'm guessing that it was standard procedure for thousands of years.

> This doesn't say what the author wants it to say.

The way I read it, the author was perfectly aware of this interpretation.