I can't be sure, but he may be referring to the meadows/black forrest interface around the spawn point in this seed, View it in "biome only" for a clearer pattern.
https://valheim-map.world/?seed=IX7JpLYpLd&offset=-539%2C-416&zoom=0.105&view=1&ver=0.202.19

See how the meadows kind of spike into the BF?