Hi, I represent the Distant Horizons team.

While this incompatibility has already been reported before, I'll be providing some additional information to hopefully be of help in making these two mods work together out of the box.

On our Discord server, we have the following entry in our FAQ:
"Q: Lod's aren't rendering with Alex's Caves
A: Go into the Alex's Caves client config (located at .minecraft/config/alexscaves-client.toml) and disable biome_ambient_light_coloring"

If the above change is not made to the user's config file, Distant Horizons' LoD chunks will not render.
What I propose is for Alex's Caves to check for Distant Horizons (possibly during startup or config load) and override the user's setting for biome ambient light.