Super impressive his custom engine is running 3 million trees x 4 million polygons, per his claims. That's 12 trillion polygons per frame. And at 60 FPS, that is 720 Trillion polygons per Second.

Following quote from author's YouTube:

A new forest showcase in the engine I've been creating for the past 8 months or so (pretty much all of it handwritten). I have been working on a lot of stuff in Godot since the last video (GI, virtual textures and other things), but I ultimately decided it would make more sense to roll a custom engine with an architecture defined by all of the things I need and not bolted onto some engine that tries to do everything. The engine doesn't do a lot of different things, but what it does (rendering a heavy amount of instances with a heavy amount of polygons) it does incredibly well.

Architecture:

    CLODs (virtual geometry) with voxels for far representations (no impostors are used here).
    Screenspace + Voxel hybrid GI (no raytracing).
    Proxies without instance data for very far away representations.

Additionally I wanted to support expressive weather systems from the ground up (since it's going to be a big part of my game). I'm not 100% there yet but I will keep improving the realism.