The definition of good in that statement merely means the book has some sort of popular appeal. Even to me, coming from the reader's not the critic's point of view, this does not necessarily equate with a "good" book. It's made into a movie because the producer believes it will make money. Popularity does not determine whether or not the story is well written. A certain segment of society is attracted to the story for one reason or another and will most likely spend the money to sit in a theater and watch an adaptation.
Words have meaning and, unfortunately, implicit in that statement is that the making of films is a higher art form than the writing of books. It may not have been meant that way, but it is what a reader would get out of the statement (or, at least, this reader). If you say a movie must be good because it was taken from book x, the book is placed on the higher ground. To say a book must be good because it was made into a movie, makes the movie the determining factor and, thus, the higher art form.
Actually, there are two different art forms present here. Just because the one is good does not make the other automatically good as well. If I write a novel based on a famous work of art, will it be a given that my book is good? Not even close. You can't make that determination when comparing two different art forms. You can't even come to that conclusion when comparing two different books written by the same person, in my opinion.
Once again I'm all over the place (I may edit this when I'm not so tired), but though the implication is film making is the higher art form, that statement actually tells me it appeals to the lower common denominator. Those who don't want to read may watch a movie based on a popular book. It gives instant gratification which a book is unable to provide.
Perhaps what it comes down to is you can't judge a book by its movie and you can't judge a movie by its book. They are two, independent, art forms which stand, or fall, alone. ***To put it another way, you can't determine the quality of a book based on the quality of the movie adaptation. A book's value is entirely separate from a movie's. Truly a book is neither more nor less estimable because there is a movie adaptation.
I will say that once they do adapt what I consider to be a good book, I have a tendency to judge the movie based on how faithful it was to that book. Since this may be somewhat unfair, I try to divide my judgment into two parts: 1-is the movie a sound adaptation of said book, 2-is the movie skillfully done.