I think we agree, but just differ slightly on what is and is not "common". Most people, don't commonly discuss grammar unless they are a teacher, editor, writer, or just generally a grammar-nerd. :D
The most common usage of "frase" is in the meaning of "sentence". "frase" can also mean "phrase" or "expression". The original point was that this translation was not accepting "sentence" which is the most common translation. This in no way excludes "phrase" as another valid translation.
Hola geneven: Oh, I see what you mean. Ok, here is the Spanish: Conjunto de palabras que basta para formar sentido, especialmente cuando no llega a constituir una oración cabal. [reference, Diccionario De La Lengua Española, Real Academia Española, Madrid]. Translation: A set of words sufficient to form a meaning, especially when it does not arrive at constituting a complete sentence.
All of the foregoing seem to miss the point that one would almost never say "This isn't a phrase" in English. If referring to one word, it's pointless; if more than one, it is a phrase. The question whether a particular phrase is a sentence is common in several contexts. Therefore the DL authors were remiss in not accepting "This is not a sentence." Period!