There is a saying in the sciences - "Extraordinary claims require extraordinary proofs" - which I think applies here.
The evidence for a date between 500 and 400 bce is strong, but this writer dismisses it without giving any evidence against it. On the other end of his argument,he makes a lot of quite unlikely claims without any supporting evidence.
Try this thread http://www.dhammawheel.com/viewtopic.php?f=29&t=4073
for more discussion.