The reason why they opted to air the Chinese dub is unclear, but as StarTimes Group was formed in Beijing in 1988 it is possible they at some point acquired the rights to this dub, maybe even aired it on a channel they owned in China between 1998 and 2000, as I've heard Dragon Ball aired on many channels during that time (suggesting perhaps it was syndicated).
More curiously, as we can hear Stephanie Nadolny's Goku in this promo however it appears at some point the channel aired Funimation's dub.
Moreover, as this promo indicates the channel also aired Dragon Ball Z, which wasn't dubbed in mainland China. Was the switch to English so viewers could get used to hearing the series in that language? Was the Chinese dub of Dragon Ball only aired to test the waters airing the series in Mandarin?

