Well, that depends on what you mean by "valid". The grammar rules that you find written down are an approximate encoding of rules that native speakers have all

Well, that depends on what you mean by "valid". The grammar rules that you find written down are an approximate encoding of rules that native speakers have all internalized. These written down rules are not perfect, and have edge cases. They allow for sentences that a native speaker would not formulate or easily comprehend. The "Buffalo" sentence is an example of such an edge case.