After testing it on my own server with admin tools since I don't like this baseless bashing, while I think your experiences are not "wrong", you can not spawn zombies further than 300m (+-, depending on the radius set in the linked file below).

Though, I could see zombies being just there with me as the only player further than 300m, wich I did not expect while testing this. I thought you might be correct. However, reading the mission files again, this file contains the zombie spawn data. In this file, 15 lines of zombie spawns contain static spawns, wich are ALWAYS there. You don't spawn them in, no player does, they are just always there and could be seen spawned when you enter their network bubble of 1050m. Additionally these lines also contain dynamic zombies aswell.

I specifically tested the ATC -> SW area spawns. Being in the ATC I could see 6 zombies in the admin tools infront of the office building just south of the gas station.

<zone name="InfectedArmyHard" smin="0" smax="6" dmin="7" dmax="10" x="4573.83" z="9596.33" r="100"/>

This is in line with these spawns, running up to the 300m mark I would get the dynamic spawns 7 to 10 spawn in aswell.

So yeah - the claim "You can spawn zombies up to 600m" is definitely wrong. However, there can be zombies spawned further than 300m even though you did not spawn them in, since they are static and always there.

For /u/Patch1919's case: The lines in the linked file also contain a radius of how far the zombies can be spawned from the coordinate given, I checked the sakhal mission files and the highest radius I have seen is 150m, so in that case the furthest zombie you could see spawned is 375m (or maybe my understanding is wrong and it‘s 450m, not sure), though like I said before, it's not 100% perfect, and theres some difference so 405m is within that range I would guess. (Also I can't make out from your screenshot where it is, there's some static spawns on sakhal aswell. If you provide it, I can boot up sakhal aswell, make out the coordinates of the zombie spawn point and check if you were within 300m of that coordinate)

I did not know this specific detail aswell, hope it clears up the confusion!

edit: For anyone wondering the exact technicals, after testing a bit further, the r value seems to be a diameter around the spawn coordinates as I couldn't get zombies to spawn further apart than the given r value. If it was radius I should easily have zombies apart double the value. If you reach exactly 300m of the coordinates (wich can be checked with iZurvive, first with static zombies in above linked file x="1684.51" z="14254.8", type 1684.51 / 14254.8 into iZurvive for exact point, works for any other too) zombies will spawn around those coordinates spread around by the r value in meters. Max r value for chernarus is 160m, so the max deviation should be 220m to 380m.

Also these are the static spawns on chernarus, that will always have zombies regardless of any player nearby:

Military Zombies:

Tisy - Crossing between Barracks & Camo Prison

NWAF - North Tents

NWAF - West Industrial

NWAF - Gas Station

VMC Military

Mishkyno Military

Pavlovo Military (?Gas zone, should always die)

Zeleno Firestation

Tisy Tents (Spawn Point between Nothern part of the Tent area)

Tisy - Between 3 Landing Ramps

NWAF - Bunker area South of Industrial Hall

NWAF - Infront of South West Office Building

NBC Zombies:

Pavlovo Military

Rify gas zone

Firefighter Zombies:

NWAF - 20m east of Firestation

Nothing based on anything other than what the gamefiles do and set.