Why would that be the case? Wouldn't sapient species tend towards low numbers of high-quality offspring?
Why would they? Why would "less" intrinsically mean "better"? We humans don't owe our sapience to our reproductive method, nor is it an evolution of something lesser from our relevant past.
Can doesn't mean did, and their engineering has repeatedly been shown to be somewhat more complicated than that; they couldn't fully disguise Thin Men in NuCOM, and apparently couldn't or didn't want to copy & paste all of their current goons' advantages into each other. That seems to leave quite a bit of room for odd aesthetic choices.
It would be far too strange (and hentai-like) for aliens to develop an aesthetic preference for human breasts, to the point of wasting scientific resources to purposefully replicate them on one of their combat species.
Actually, the rightmost image shows them trailing up towards the head suspiciously, which strongly suggests venom sacs to me.
That doesn't mean they couldn't have been mammary glands just because of scales or genetic engineering, and it's unlikely to do a great deal against my demented headcanon, though.
Can't judge your headcanon, of course. Just saying mammary glands don't make any sense for this species. Could it still happen? Yes, but I don't find it plausible, personally.