Hypothetically speaking, if a mod like this did exist would it work on SP games also.
I have only seen it mentioned having to do with multiplayer servers.
Depends how it is made. Some server side performance mods for example, "improve" performance by reducing server to client netcode updates to preserve CPU utilisation. On single player games this is unlikely to have any much of a performance impact.
However, Grandpa Minion mentioned that the mod had to do with utilising more cores. If so, and you have more than 4 cores in your PC, this is likely to help. But writing efficient multi-threaded applications is difficult enough when you are writing it from scratch, let alone modding.
Imagine driving 4 cars down a freeway, when each car needs to communicate with the others in order share snacks, directions, toilet breaks, and fuel stops, and no car is allowed to get too far ahead or behind of any other car. That's multi-threaded programming. The whole convoy moves at the pace of the slowest vehicle - and then some inefficiencies on top.
You'd think it's fairly simple to get two different zombie AI's to take a thread each, but each one has to figure out what path it wants to take, considering the other zombie's path (so they don't both try to share the same space), and they need to keep checking if the blocks are still stable, if they are being hit by traps, bullets or players, and if they are still heading towards a valid target.
It is far easier to give each Zombie an identifier, and then have a single process sequentially decide the actions of each Zombie, because now that a single thread is control, you can make decisions like "since zombie 10 has already moved and occupied that space, Zombie 11 needs to repath around it". Often when programmers do this, they will divide the game up into "ticks" and then give each Zombie a time budget so that they can fit all calculations within a single tick and make the game look real time.
This is why I'm very skeptical of Grandpa's claims. While I don't doubt some performance tweaking by mods can be done, it won't be by multi-core optimisation (not to mention that in practice, if you optimise for multi-core, your CPU utilisation should go up, not down as you are now using MORE of the available resources, not less). If the mod exists at all, it looks like efficiency optimisations, not multi-core optimisations.