I thought this was showing Voldemort mocking Lilly - he agreed sarcasticaly, since it was obvious he would simply kill Harry next if she surrendered - but accidentally fulfiling the requirements for a ritual:

"You see, Mr. Potter, the chant of every ritual names that which is to be sacrificed, and that which is to be gained. The chant which you gave to Miss Davis spoke, first, of a darkness beyond darkness, buried beneath the flow of time, which knows the gate, and is the gate. And the second thing spoken of, Mr. Potter, was the manifestation of your own presence. And always, in each element of the ritual, first is named that which is sacrificed, and then is said the use commanded of it." -Chapter 74

I assumed this was meant to allow events near-identical to canon without Quirrelmort seeming incompetent enough to simply forget about the vast magical power sacrificing yourself for love provides. Indeed, I suspect such a resource does not exist in the MORverse, both because it privileges love - a fairly unremarkable neurochemical state - and because its just too easy to exploit. It seems out of place, somehow.