Strongholds are able to generate in the_void, the one and only biome meant to only house the void_start_platform feature and nothing else.

How to reproduce:

1. Create a Single Biome world with The Void as the selected biome.
2. Throw an eye of ender or run /locate minecraft:stronghold
3. Observe how the eye of ender points in a specific direction or how the command shows you the coordinates of a stronghold.

Cause:

In 22w07a, all structure features are configured to generate in biomes of a specific tag, one for each structure. Strongholds use #minecraft:has_structure/stronghold, which contains the value "minecraft:the_void", and thus strongholds can generate in that biome.