The hex between Bangkok to Rangoon became transport route after the the event "Bridge on Kwai river card #18" .
Please specify the rule that stated hex 2008 to 2110 are NOT transport route before the event. (map says only special event)
You might think a nitpick but I want a material that can refute to
opponent who claims the rule book doesn't state the NO transport route.
I think you have the hex numbers wrong, but I understand the question. Hexes 2108 and 2109 are not transportation routes prior to the play of the Bridge over the River Kwai. The rule that you are looking for is on the map in small type it says River Kwai and the two hexes say Special Event on them. If you are asking if there is any reference to this in the rules, the answer is no.
I hope that helps,