Managed to answer my own question and it's literally the line below the one I asked about.
"As shown in Figure 107, all the L2 bridging instances defined on a given DLR would be active on the ESXi host in edge rack 1 where the corresponding DLR Active Control VM is running. That said,it is possible to create multiple sets of bridging instances and associate them with different DLRs, so to be able to spread the bridging load across different ESXi hosts"
I've only just got my morning coffee too, guess I should have tried drinking it before reading the design guide!