You're exaggerating how dangerous BAs would be in this situation. As far as I know, useful single crystal boron arsenide is very expensive to make in industrial quantities and it's too brittle to be used as a large heatsink - at best you can grow a several micron thick layer of it on a semi-flexible substrate like for solar panels. Instead, I'm guessing their design uses small quantities of single crystal BAs to interface between the wafer and the rest of the chip package so it would better transfer heat from the internal silicon to a regular heatsink. Thermal conductivity at that interface is a hard limit to how fast the heatsink can draw heat from the chip package and since its contained, toxicity is largely a concern for recycling.