Thank you for your post, it is helpful but we should try and stick to the Quran for our understanding of salat.
Many books try to denigrate Islam which is said to have copied everything from the pagan Arabs and other cultures, but they are using the same unreliable 200 year old, at least, sources to prove their point!
They claim 2 prayers were done, so the book shows that there were sources not confirming 5 times namaz and puts to rest the '1400 years of no disagreement over namaz amongst muslims and this denial of a ritual prayer is a modern invention!, which we frequently hear from the namazi's. Reading Bukhari is just as unreliable a source on what should be done in namaz and he had chapters called Book of Prayers!
The Quran is clear, salat is from dawn to dusk, that is from the time you wake to when you sleep, following the commands of Allah in everything you do.