I doubt if any one can tell you about the authenticity. I as a Hindu have never even heard of the first book and not even 1% of the Hindus would have read Manu smriti.
Stop quoting from blogspots like "Hinduism Exposed". They are out of context verses. IF you are curious you should go find the books and read it yourself. There a millions other similar blogsites and websites on the lines of "Islam Exposed", which has some pretty strange verses. Should I start asking you about the authenticity of those verses?