She is actually straight up converting the food to Stormlight. It's not like Allomancy, where the metals are the gateway to the power, she is just converting directly.
Resonances form when you have a power and if molds slightly to you, just your flavor of that magic ability. Generally if you have enough powers to have some from multiple Shards (excluding the Shards that have melded their Investiture together [willfully or not] to make that magic), you don't get a resonance because the powers don't get a chance to resonate with your Cognitive/Spiritual selves because there's so many of them. No subtlety in, say, a Mistborn, with 16 powers, so no resonance. Twinborn can and do have resonances, but going much further than 2 powers like that seems to cross the threshold that we've seen.