Greenspan is saying that STEM workers shouldn't make more than other workers. The argument isn't for increasing worker supply from external sources in general,

Greenspan is saying that STEM workers shouldn't make more than other workers. The argument isn't for increasing worker supply from external sources in general, it's for targeting STEM for more visas. It's known that students find STEM harder in school and STEM jobs are widely considered more difficult than other jobs, thus earning their wage difference. Like it's been said by others, this will result in less STEM degrees domestically, the exact opposite of how this problem should be solved.