I have a part 1 and part 2 reply.
Part 1:
My statement you quoted above was super 'hand wavy', but it is a mass & energy balance situation.
Ethanol production from corn, corn stover, or whatever (you can tell I'm not a farmer), removes carbon & energy from the "farm field system" that would otherwise stay in/on the field, be tilled or decompose into soil, and end up in next years' corn (or the year after that, etc.). It makes the farming cycle less efficient from a carbon management perspective and, as others have mentioned, that means more fertilizer, etc. to make up for the carbon loss. Without increased fertilizer use and general land management... crop production would decline year over year as result of removing plant material for ethanol production.
Also... one could imagine corn breeding and propagation scenarios in a 'field corn' field where (a) silage is maximized for feeding cows and (b) a trade off between silage and stover is made to maximize overall profit.
So there does end up being a competition between (1) optimizing for efficient food production and soil management and (1) optimizing for maximum profit. The second option does diminish food production unless made up for by there means.
Still not a great explanation. I don't have any references to share but there are some good review articles out there that talk about this in detail.
Part 2:
I was "sort-of" conflating corn with other crops for biofuel production. I should not have done that. My statement that ethanol production from corn "places additional competitive demand on our food supply" might not be completely true in a high-impact sense. But ethanol production from corn does still impact food production and land-use as described in part 1.
In the case where a crop like switch grass is cultivated specifically for biofuel production... here we would definitely have a case where there is competition for use of arable land, a finite resource, between biofuel production and food production.