Do you see that as being the same as having a "-" in the Horror check/damage part of the monster chit? IIRC there are Mythos cards that increase "...sanity damage (if any)". Line is not the same as having Sanity damage of 0 in this instance, so why would Overwhelming/Nightmarish be treated differently?
Just to be clear about what I was talking about, I was replying to the statement that stated:
if they had said 'increase" their Overwhelming by 1' it would have no effect unless the monster already had Overwhelming.
(I think you understood that, but I could see other readers gettings confused.)
As far as the original poster's question, I've always interpreted it the same way as ColtsFan described.
Now, as far as your question, didn't you answer it yourself?
"A line is not the same as ... 0."
Anyway, as long as the rules don't actually read "increase", there's not much point having a long, involved discussion about it. To be honest, I suspect that if they did word a rule that way, they would also clarify explicitly whether this turned non-overwhelming monsters into overwhelming monsters. For example, by saying "Overwhelming monsters have +1 Overwhelming." (I've used almost if not exactly that wording myself on a card.)