Maybe it was poorly phrased - land available for agriculture is small relative to their population. Iceland is huge but largely barren, Japan is even bigger but feeding 127m people is probably not possible with the land they have available
Except that it is. Japan is self-sufficient for food. Land marked for growing rice is essentially "eternally locked" to growing rice; you cannot sell it or use it for other purposes, and are required to plant it.
There are a lot of subsidies for Japanese rice growers too but I think this is primarily because without them they would not be competitive with American imports.
Also, the issue of US rice exports was one of the biggest stumbling blocks for US-Japanese TPP negotiations, so I don't think Japanese rice producers believe Japanese people will not eat medium-grain rice.
It would be more easy to do if they stopped promoting single farmer work, and actually made an industry of their agriculture. One person taking care of a small piece of land is ridiculously expensive and that shows in the price of fruits and vegetables here - way more expensive that about anywhere else in the world. (also because the restrict importations severely, thanks to the JA Mafia).