I think that you misunderstand the line. Actually it is said: "If file file can't be read or is empty":
<pre>
$ touch /tmp/empty_file
$ echo "HelloWorld" > /tmp/filled_file
$ if [ ! -s /tmp/empty_file ]; then echo "IS empty"; fi
IS empty
$ if [ -s /tmp/filled_file ]; then echo "Is NOT empty"; fi
Is NOT empty
</pre>

I suppose this is the comment which is confusing. I'll change it