I think
Ghost Recon
Wildlands and
The Witcher
3 have similar foliage density. TW3's is obviously nowhere near the quality of Horizon though.
I have recently played the beta of GR:W, but not on ultra so can't really say about the same experience i had regarding the vegetation, as shown in the Images you have posted, but during my time in beta , the density of the vegetation is good and it will fill your camera quiet nicely but still The major difference is Variations from trees to shrubs to flowers to Creepers to rock moss etc etc along with the density.
Even check the LOD(vegetation) as further away from the camera in GR:W , and check in the images i quoted, there is quiet a lot of difference in the Density and variations and hence i will give the props to Horizon here. its truly something else because of the composition of the whole vegetation system.Its Truly Breathtaking.