-duino typically means that it's physically drop in compatible with Arduino devices, so you can (physically) connect it to anything you'd connect to an Arduino without needing to make any (physical) modifications.
Of course, you still have to make the FGPA do something. An advantage of an FPGA over an STM32 would be if you're willing to invest some time in hardware description languages (HDLs) like VHDL or Verilog to design your own system, or want to drop in novel cores (like RISC-V) that aren't physically available yet or are still in a state of flux. RISC-V isn't fully specified yet so a "soft" core version on an FPGA offers you the ability to upgrade your device without needing to actually purchase a new one, and remove and replace the old one.
In theory, an Arduino compatible FPGA-based device could be used to develop applications totally sans software (that is, all the logic is encoded in the HDL) as well, which can be advantageous in prototyping, and maybe in performance depending on the FPGA in question.
Of course, you still have to make the FGPA do something. An advantage of an FPGA over an STM32 would be if you're willing to invest some time in hardware description languages (HDLs) like VHDL or Verilog to design your own system, or want to drop in novel cores (like RISC-V) that aren't physically available yet or are still in a state of flux. RISC-V isn't fully specified yet so a "soft" core version on an FPGA offers you the ability to upgrade your device without needing to actually purchase a new one, and remove and replace the old one.
In theory, an Arduino compatible FPGA-based device could be used to develop applications totally sans software (that is, all the logic is encoded in the HDL) as well, which can be advantageous in prototyping, and maybe in performance depending on the FPGA in question.