Let's just hope The Author would soon atleast share the source codes for both smod and smod2 in the future so someone can make additional improvements and probably make a full mp mod out of it.
I kinda figured out how bullet time is kinda made, it actually changes "host_timescale" to a number supplied by "bullettimesim_timescale" (in this case, 0.2, or 1/5 of the normal time) and after bullet time is done, switches "host_timescale" back to 1 (normal speed). The bullets are created if bullet time is enabled and only if bullet time is enabled, probably using a check if we are in bullet time or not. Kicking is using a extra view model created in "player.cpp" in the HL2 Mod code, and a delay in the view model's events, telling when to apply the damage from the kick attack. The custom weapon system is just using a key values system modified from the default system used to load scripts for weapons. The model replace system sets the npc's model by reading key values from npc_replace_model on spawn, Random Models Key Values
. When you think about it, SMod's code is really easy to replicate (though with the fixes, it might take some time), seeing how most of it's stuff (as previously said), is in Valve's Dev Wiki, even if it's not, it's actually easy to figure out if you can examine and use logic to make up the code of special features for SMod. The way bullet time is made in some Garry's Mod (can't believe I just said that name here) addons have actually done bullet time exactly how The Author did his bullet time.
So in a sense, anyone can remake SMod at their will if they have the time and knowledge of C++ to recreate it. Though, it would more easy if The Author would release his code.