Not clear from their post but interesting whether that is 16Kx16K tiles with streaming or 16Kx16K images being streamed in to the virtual tiles. That is so beyond what is useful even on a super high res game though - you don't really gain anything beyond 8K tiles and 4K assets for gaming purposes - even when people have higher than UHD resolution displays (if done properly).
Sounds like 16K textures because streaming shouldn't cause crashes.
Each 16K texture (uncompressed) requires ~1GB VRAM so it's not surprising he maxed it out quickly.
http://vterrain.org/LargeTextures/
I get the feeling it's for a professional app because for games you'd create landscapes with Unreal's terrain tools and paint grass on to it and it's very efficient in regards to VRAM rather than using your own images.