I can't tell you by exact knowing it, but after translation
of that into "CD's internal brain_language"
It sounds to me that way:
without -nomap:
when starting the game, ofp creates a reference map
to all addons being loaded, which will give you access to
all addons being included to that map
with -nomap:
a technique (as you assumpted above) will be used, without
creating a reference map, so that addons can be accessed
only when needed.
Maybe in case of small missions with only very few addon's usage this will not make any difference to the user, but the
more complex it becomes, the more performance issues you
will notice (or not).
:note - the statement above is not aproved nor is it supported
by anyone else - it's just what me thinks about the situation.
~S~ CD