To elaborate on what Dan said, the T2 chip includes Apple’s own NVMe PCIe SSD controller, which they developed in house. Like most SSD controllers, it uses multiple channels (maybe 8 in this case?) to address the NAND flash dies and increase performance through parallelism. Both the T2 and the NAND flash packages are soldered directly to the logic board. It is also likely that the drive can tolerate the failure of individual blocks and pages, or perhaps even whole NAND dies by using parity data.

This is a different concept than RAID though, which employs Redundant Arrays of Independent Drives to increase availability and in some cases performance. RAID 0 in particular ditches the redundant bit and stripes across an array of independent drives purely for increased performance. Apple decided to put the NAND flash packages for the iMac Pro on a pair of removable modules, however they were in no way independent drives. There was still only a single SSD controller located in the T2 chip on the logic board.