As far as I believe, you can set the number of rays yourself in UE, just like in an actual raytracer, say V-Ray or Arnold.
I think they meant it will shoot less rays in the same time as it cannot compute them as fast. So if someone wants to play actual games with RTX on a GTX then he will have to set the raytracing quality lower to have a playable framerate. At least that’s how I understand it.
And now to wait for the driver to be released…