No, lawn weedkillers don't kill grass, any grass. However, if you're noticing a different grass in the lawn, it's unlikely to be couch grass - that usually gives up with regular mowing, but other weed grasses don't. You can dig those out though.

Wish that was true, but the couch in our 'lawns' has been there for the 20 years we have been mowing it and it is still there and it is couch.

Oh how I wish...Ours creeps out sideways into the borders and is a real pain.