I stumbled upon this phrase in this video's description.
A few words about this masterpiece: Brahms began composing this piece in 1854 and finished it in 1876. The reasons for delaying finishing it are probably two. First, Brahms' self-critical fastidiousness, and secondly, the expectation from Brahms' friends and the public that he would continue "Beethoven's inheritance" and produce a symphony of commensurate dignity and intellectual scope, an expectation that Brahms felt he could not fulfill easily in view of the monumental reputation of Beethoven.
Is this grammatically correct? Can a numeral be a predicative in plural stating the number of the objects that are denoted by the grammatical subject? Could one say, for example
The pens are two, not three.
There are two pens, not three.
If the sentence from the video description is correct English, while my example is not, please explain the difference.