The Offset Tracking operator estimates the glacier velocity through image coregistration. It tracks the target pixel positions in two images and estimates the velocity from the range and azimuth offsets of the target positions. If SLC image is used for offset tracking, then the range offset obtained by image coregistration is in the slant range direction and it is not the offset in the ground range direction. Therefore, the velocity estimated using SLC image is not correct. You should always use GRD product for offset tracking. If no GRD products are available, you can convert the SLC products to GRD using the “S1 SLC to GRD” graph in SNAP. Then you can apply the DEM Assistant Coregistration to the converted GRD products. The graph you provided above does not have SRGR processing step in it, therefore, it does not convert the slant range image to ground range image. The images used in the offset tracking are still slant range images.