I don't consider these artists, like Jan Van Eyck, to be cheaters on their art talent, even though it was a disappointment to find out that no artist has been able to depict a precise and perfect picture with only the aid of their eye and talent. I still consider them great artists by being able to find easy solutions like lens and mirrors to acquire a precise image. This shows how much they deeply studied art; how much effort they put into these paintings; and how perfect they became to be. furthermore, when Hockney discovered that the pictures done by these artists were all measured in a very mathematic way, shows that these artists studied the math branch also to acquire more perfection in their paintings.
Furthermore, their talent is really shown within their technique of using paint over these sketches. Even though they copied the sketch, the painting was done only by them. the techniques that they used to paint the artwork shows how talented they were; not everyone can paint a piece of fabric to look so real like these artists were able to do. the use of pigments really brought out the paintings. We are able to see a very realistic light and shadow in these paintings, thanks to the use of paint.
We shouldn't forget that all artists have taken inspiration from their surroundings and have always tried to depict what they see in the most perfect way. Also, the artist always has an influence whether it be realistic or unreal, so we can't say that they are also cheaters in their artwork and talent.