According to the NTSB report, the boost system on the MD-83 only activates to assist with large nose down inputs. Because the elevator was already jammed fully nose down, there would have been no significant difference between the boost commanded position and the actual position of the elevator when the boost came on.
Analyzer of plane crashes. Contact me via @Admiral_Cloudberg on Reddit, @KyraCloudy on Twitter, or by email at kyracloudy97@gmail.com.