That post refers to finding all user-defined markers, that is, markers placed by players in-game in the map screen (which follow a naming convention), rather than by the mission-designer at design or in a script (which have arbitrary naming). To find all the markers placed the player in SP, you could use something like this:
for "_i" from 0 to 1000 do
{
_pos = getMarkerPos (format ["_user_defined #0/%1", _i]);
// Do something if _pos is a valid position.
};
I think what the people in that thread could have been talking about was that in OFP 1.75 you could make your own dialogs or use onMapClick to find out where players wanted to do something. I assume before this was available, mission-devs used to have to find a marker being placed and use that as the location. I have no idea about this though, since I came to scripting for ArmA very recently. Perhaps one of the old-guard can clarify this issue?
So anyway, I don't think this technique will work for you in any way. As I said, if you, the mission-designer, placed the markers, then they will be named and you can easily find them. If you've placed a lot and don't want to manually make a massive array of their names in order to find them, then you could follow your own naming convention, such as "myMarker1", "myMarker2", etc. and use a similar loop to find them all.
for "_i" from 1 to _numberOfMarkersPlaced do
{
_pos = getMarkerPos (format ["myMarker%1", _i]);
// Do something at _pos
};