In Ambedkar’s, Buddha and his Dhamma Siddhartha becomes a monk without seeing those signs (an old man, a diseased man and the dead). Did he see those signs or was it a cooked up story?

Yes, this is false story. The story is told as the reason for Siddhartha undertaking Parivraja.

It is completely absurd to believe that a human did not witness old man, sick, till the age of 29. Moreover to support the fact, it is enough to remember that he knew his mother died when he was born.

Dr. Ambedkar called the story as “absurd” and decoded in his book “Buddha and his Dhamma” as follows

If he took Parivraja as a result of these three sights, how is it he did not see these three sights earlier ? These are common events occurring by hundreds and the Buddha could not have failed to come across them earlier. It is impossible to accept the traditional explanation that this was the first time he saw them. The explanation is not plausible and does not appeal to reason. But if this is not the answer to the question, what is the real answer ?

The real answer is, Siddhartha took Parivraja as a result of conflict with the Sangh over the issue of water conflict between Sakyas and Koliyas