MC wanted his quiet lunchtime, but by helping the FMC and not doing what likely every other guy before had been doing (confessing to her), he unintentionally got her interested in him. We see here that she's already quite comfortable around him, and this light bantering will be ongoing.
A little different from the LN/WN where he asked her to get him bread to thank him, but she gives him a bento instead.
MC overhearing rumors that are going around about FMC. He will have the expected response of trying to get FMC to not hang out with him anymore, but she firmly puts her foot down and basically says, "Screw what other people think, I decide who I want to hang out with." MC rethinks his position and then accepts having her around during lunch, as, despite his quiet time being disturbed, he doesn't mind this setup.