Well, the sentence in question is in reference to the code given just above it. The code contains numbers.length. It does look like a variable to me. Indeed, if you had a class with an instance variable or a "field" named length and a variable named numbers, this is exactly how you would use it i.e. numbers.length.
I was thinking that you are objecting to the
word variable for length because it cannot be changed. Now it looks like you are objecting because it is not defined like int length = 5? May be I'm not understanding you correctly.