This doesn't make much sense to me. 1080p60 means the system has an output of 2 million pixels every 16.6ms, which results in ~124 400 pixels per millisecond.
4K30 has an output of 8 million pixels every 33.3ms, so it's 248 800 pixels per millisecond.
4K30 is processing twice as many pixels as 1080p60, so claiming the computational effort is the same between the two doesn't make much sense.
Besides in this case, you already know that RT is halving performance, because the game can run at 4K60 or 4K30+RT.
The performance hit with raytracing is supposed to be a known variable, and in this case it's 50%. Which is why it's very strange that they're reducing the resolution by 75% to enable RT at 60FPS.