Nothing about featuring former and contemporary enemies, Japanese and Russians, in this letter.
You cannot claim that the progressive ideas of Trek came out of one of these studio heads who considered "The Cage" to be too cerebral and wanted Trek to be more of a space western.
Pretty sure the "cerebral" thing is a bit of a myth as well. The pilot was well received by the executives. They did have problems with some of the casting (Like GR casting his then mistress) and the character of Spock. Who, ironically would be the shows break out character.
As for casting Japanese and Russian characters. First I don't think Sulu was Japanese in the beginning, but was "pan Asian". He became more Japanese later one. Secondly the war had been over almost two decades by the time Star Trek was made. American servicemen ( like my father) had been living in Japan since the end of the war, bringing home Japanese culture and "war brides". I spent half my childhood living in Japan. Japan was also our strongest Far Eastern ally and a staging point for operations in Vietnam and electronic intelligence aimed at the Soviets and Chinese.
On the Russian front, we had Nina Talbot's character in Hogan's Heroes and Illya Kuryakin in Man From UNCLE. Both predate Chekov.