Yes and no: yes, it's real iperf3 on the OpenWrt box, but no, it's a loopback test with both instances of iperf3 on that OpenWrt box. This bypasses the NICs and their drivers, and gives a rough indication about the performance of your router's network stack. As with any synthetic benchmark, huge caveats apply, but it is useful as a gross gauge as to potential (say, if your results show a given device can only touch 900 Mbps internally, there is no way it's going to give you 1 Gbps on your shiny new FTTH service).
Here's a whole thread dedicated to running the loopback test on various hardware: