求prep sc-2-34中代词指代问题

    Besides adding complementary flavors to many foods, hot sauces stimulate the release of endorphrins in the brain, just as exercise does, and these have a pain relieving effect like morphine’s.

ＰＲＥＰ语法笔记里说：代词this, that, these, those不能单独用作主语，而且选项解释成these在此处指代前边出现的endorphrins，为什么此处的these就单独当了主语了呢？