Gigabit speeds are guaranteed over ethernet, not WiFi. Try the test over an ethernet connected computer.
For that matter, 600 Mbps over WiFi5 is pretty darn good.
But I should still be seeing closer to 1,000.
Iphone 11 pro is wifi 6.
600 is not the complaint.
Not getting what is paid for is the complaint.
You said you have an XB6. The XB6 has WiFi5. Your connection will be limited by the slower protocol.
I have an iPhone 11 Pro Max, and it's limited to 550Mbps over WiFi using an XB6 for precisely the same reason.