Ah, WPSP doesn’t use
the_excerpt() function, as doing so would mean the excerpt length you set would also apply to your regular blog posts (which use the same function).
So instead, we use the
wp_trim_words() function, which allows us to set the excerpt length, read more etc.. only for posts in WPSP without affecting other posts throughout your site.
Of course, if you needed to use
the_excerpt(), you could fork WPSP and make that change yourself for your own version: https://github.com/tomusborne/WP-Show-Posts