Question: I'm wondering if it's possible to use the isp1510 in TDOA applications or if it's only for 2 way ranging.

Answer: In theory, the ISP1510 could be used as the Decawave chipset for TDOA application by using the SYNC pin to synchronize all the anchors.
Unfortunately, we don’t have at present this SYNC pin routed to one of the module IOs (as you can see in the module schematic from the datasheet).
We are scheduling to add this IO in the next release of the module, this will occur in the next couple of months. In between time you can test the TDOA functionality directly with the chipset.