To be a virus, it needs to be executable in one form or another. An executable is anything your computer or a program on your computer executes as code. This could be a .exe, .bat, .dll, .msi or various other extensions. There are other types of virus which do not rely on executable code, but instead rely on the power of the program running already. A good example of this would be macro worms executed inside MS Word or MS Excel. The worms had no power, unless opened and run inside the macro program. They didn't do much though, except spread, like a worm.
Factorio mods, on the other hand, is largely created with a scripting language. While it could certainly do some crazy things, it cannot (CANNOT) do anything that factorio cannot already do. A factorio mod could easily wreck havoc inside your factorio world, it could corrupt your save data, it could probably even mess with enough code that you might have to reinstall factorio.
But it cannot do anything that factorio cannot already do. Factorio cannot execute code that spreads through your system, so a factorio mod could not do that either.
As long as you are only downloading the .zip file from this website, and all you do is deposit that file inside your %appdata%/factorio/mods folder, there's nothing that can hurt you.
