You are the 2022 poster lawn for spring seeded lawns. You didn't mention what seed you used, but you can be sure there are spring/summer weed seeds which sprout faster than your grass seed.

You could try spraying something like Weed-b-Gon on the area and hope it doesn't affect the new grass. Or you could accept the weeds as your lawn for the rest of the summer, and in August start the reseeding process with RoundUp to kill off the weeds. Just mow when the weeds start to look shaggy. By waiting until after the summer heat fades away, you should not have the same problem with the summer and spring weeds. Also there is nothing really wrong with having the weeds growing there, because they are giving you roots in the soil. When the weeds die they become the organic matter that everyone wants to have in the soil.

By the way, for future reference, even though you read a lot about how to prepare the soil for new seed, you should not have rototilled the soil, because that destroys the soil structure and turns it into fluff. The idea or rototilling is relatively new. If you go back to the middle of the last century that concept wasn't around.