I suppose it boils down to what we are using our sight to do. Are they only being used to read a recipe and then try to measure things to match exactly? Or is it possible for sight to involve something more than merely seeing? Maybe the word I'm looking for is intuition. Experience. Trial and error, until you can see the right measurement without necessarily taking out the standard cups and spoons we tend to rely so heavily on here in the States.
Cooks throughout history and across the world have been able to make do without using perfect measuring cups and spoons. It's a skill that has to be taught through experience, whether in culinary schools or, more often, in the kitchen with one's mother and grandmothers. When you see the ease and grace with which an experienced cook can prepare a dish, it certainly makes an impression.
They cook by sight.
It was a concept that frustrated me for a long time. I would ask my husband's mother for exact measurements so that I would be able to recreate my husband's favorite dishes. But all she could do was shrug and tell me that she does everything "na glaz", or by sight. This was a skill that took a long time to develop. It took a while for me to even attempt, honestly.
But it works. I'm still honing my skills (my "sight"), so please make sure to take the measurements with a grain of salt. Experiment. Enjoy the cooking experience!