This name change is incorrect and needs to be reverted. Official Metrolinx documentation addresses this stretch of track as the "GO Subdivision".

For example, the introduction on page viii of this document addresses it as such.

https://assets.metrolinx.com/image/upload/v1663237565/Documents/Metrolinx/2020-04-29-Bowmanville-Rail-Service-Extension-PDBC-Final-v3-1.pdf