There were two reasons the Court of Appeal hearing held that the complaint could be heard in UK courts:

1. They relate to alleged harm caused by decisions and policies made centrally by Dyson UK companies and personnel

2. There was substantial risk that they would not be able to access justice in the Malaysian courts

Both seem reasonable. The UK personnel may have engaged in an activity they knew were illegal. Foreign citizen can generally sue in another country, if they must establish that the court has jurisdiction over the matter -- which they seem to have done.

If anything, it should make the anti-slavery mandates of manufacturers, particularly fashion, sit up straight.