It's an Arduino with an FPGA on it, yes. But the FPGA is practically a hood ornament: there is no support whatsoever for building gateware in the Arduino IDE, only for using a set of prebuilt demonstration bitstreams, and what little documentation Arduino has provided on using the FPGA (e.g. https://docs.arduino.cc/tutorials/mkr-vidor-4000/vidor-quart...) is extremely vague and is missing a lot of critical information.
(Additionally, some of the product specifications for the board are highly misleading -- for example, some pages refer to the edge connector as "mini PCI Express" and suggest that it can be used "to creat [sic] your own PCI interfaces", but the FPGA on the board does not support PCIe.)
You're much better off with a dedicated FPGA development board. Arduino hasn't brought anything useful to the table here.