The r in your ... exec r uboot_a reset command stands for “read”, which just copies data from your phone to the computer. You seem to be looking for a “write” command to flash the partitions. As explained in Flashing the Jolla C2 ... | SailfishOS community wiki you need to use the w command for that, e.g. like this:
./spd_dump \
exec_addr 0x65015f08 \
fdl fdl1-dl.bin 0x65000800 \
fdl fdl2-dl.bin 0x9efffe00 exec \
w uboot_a SailfishOS/lk.bin \
w uboot_b SailfishOS/lk.bin \
w dtbo_a SailfishOS/dtbo.img \
w dtbo_b SailfishOS/dtbo.img \
reset
The thing after the partition name (SailfishOS/lk.bin) is a path to the file you want to flash to the partition. If your lk.bin file is somewhere else, you need to adapt that part.