The GRX 400 shifters (just like the latest versions of Tiagra) use 11 speed technology in the rear derailleurs, but just pull more cable to shift to ten speed (than Shimano's 11 speed shifters do). Therefore the 10 speed Tiagra and GRX 400 can use 11 speed rear derailleurs and vice versa. But it's not officially supported.
Where did you see the RX400 cable pull is the same as 11 speed? That's not reflected in the Shimano compatibility chart (unless I'm reading them wrong)..