Penises crop up in evolutionary history a lot but at the same time it's weird they would exterminate an entire sex. Plus I could easily see them adding whatever organs they needed to be sexually compatible with natives. So I'm guessing they started out with a different arrangement of sexes. Don't need traditional hominid peg in slot to have ejaculation and orgasm, necessarily.
Perhaps, but bear in mind that:
1.) "Peg in slot" is not merely hominid - whales do this, as do at least some insects, it is a fairly widespread feature
2.) Wiping out an entire sex is less weird than you might think in the Earwaverse, consider the Whale-mothers, who are essentially lobotomized baby machines and alive only because the Dunyain lacked the Tekne and could not produce full cloning to eliminate women entirely. It is entirely likely that the Inchoroi did not make tools of wombs as the Dunyain did only because they had to tools to replace wombs altogether.