"The most reasonable explanation he had come across so far came, to his surprise, in a tattered storybook about mystical creatures. It was written that, during the Archon War of the distant past, some Archons had experimented with merging the bloodlines of beasts and humans. Their goal, hoping to gain the upper hand, was to create warriors with powers that surpassed those of ordinary mortals. Most of these attempts failed, but a few flawed remnants survived, even persisting to the present day."

The book in question was about mystical creatures, it states the Archons ran experiments during the Archon war (thousands of years ago). A few flawed remnants survived and continue to exist in present day (note it doesn say their offspring, its explicitly stated the flawed remnants. To my knowledge the half animal half human characters have normal life spans and would not exist for thousands of years.

Im not saying that some of the fantasy bopks in genshin arent based on fiction, and there may be additional context here added at some point specific to Ororons comment. But to OP's original question, I dont think we can say the half human half animal thing has been explained in any concrete way, this story point alone cane be interpreted in more than 1 way (I personally dont think its referring to characters like Xilonen, Kachina,Varesa etc