To me, the whole manifest ID fuzz makes no sense. There are a dozen manifest IDs in the players appmanifest_377160.acf for the dozen FO4 main files as main game, DLCs PLUS english depots of all that PLUS your chosen language of all that. <- Old entries are not important. They should be correct anyway.
WHAT manifest ID and WHERE replacing is the thing we want to know. Btw. every ID/manifest in that file fits to the depots of SteamDB...
Did you mean the manifest entry of the NEW UPCOMING DLC when it comes out and before people launch steam? <--- YES
Thats what thread creator meant:
The only thing that makes sense to me is the part about the file's read-only, but only after we added the entries of the new DLC. Otherwise steam will constantly try to update FO4 after the update is released. <- YES, exactly that's how it works.