This reads like it's relative to Dimorphos
This article is something you can probably understand but it's WAY over my head. But I did find this in the Introduction section:
With its current trajectory, the spacecraft will impact Dimorphos's leading hemisphere with a relative speed of ∼6.15 km s−1 on 2022 September 26
According to Google, 6.15 km/ps = 13757.16 mph. (or the approximate 14k mph reported in the press.)