What you described are nutletts from the purple nutsedge. They can lie dormant for many years and you stirred them up by tilling or bringing them to the surface (or maybe not). You did right by tilling but it's really not your fault cause your damned if do and the same if you don't. They were there already (dormant)and the glyphosate won't get them all. Certainly, all the extra water being applied to the new lawn helps to bring them up too.
The customer will need to further treat with sedgehammer. You can't perform miracles on a lawn that was full of weeds and neglected. Again, how can you be responsible for future weeds that have been previously dormant or weeds seeds present not yet germinated?