That's ridiculous. Let's take a long view on China from Japan's perspective:
* Han Chinese displaced people in every direction, throughout what is now China and even into Malaya.
* Chinese moved south & west, killing native peoples. (numerous Mongol, Dzungar and Tibetan elimination campaigns)
* China takes land from Tibet (1930-1932).
* "Chinese unity" includes a lot of places that aren't Han Chinese. (routinely interfering in Mongolia)
* China previously demanded tribute from Japan.
* Republic of China oppressed the Manchu, killing hundreds of thousands.
* China wanted to kick Japan out of the mainland.
It was only logical to feel threatened by China. If properly unified it would outclass and defeat Japan. They had a momentary advantage and felt compelled to take action to protect themselves against a hostile China.
Most nations have many reasons to fear their neighbors if you look far enough back.
The situation dictated policy. Either their war in China ground to a halt or they conquer SEA.
I don't think the "Manifest destiny" entered much into IJA/IJN thinking in 1941.
* Han Chinese displaced people in every direction, throughout what is now China and even into Malaya.
* Chinese moved south & west, killing native peoples. (numerous Mongol, Dzungar and Tibetan elimination campaigns)
* China takes land from Tibet (1930-1932).
* "Chinese unity" includes a lot of places that aren't Han Chinese. (routinely interfering in Mongolia)
* China previously demanded tribute from Japan.
* Republic of China oppressed the Manchu, killing hundreds of thousands.
* China wanted to kick Japan out of the mainland.
It was only logical to feel threatened by China. If properly unified it would outclass and defeat Japan. They had a momentary advantage and felt compelled to take action to protect themselves against a hostile China.