By Horizon period, it means the open documents within this Horizon period alone will be considered, if it is a Sales Order & if the material availability date falls within this 15 days period, yes they will still be considered even if the requested delivery date is 2 months later.
your question is little tricky, yes at the time of sales order creation if the material availability date was after 15 days & the status had got updated as not performed , u still have a chance of that order going unnoticed, but at the time of delivery it might be looked upon( provided credit management was made at delivery level). if you dont have the CM at delivery / PGI level, your question is correct.
but i read somewhere, but not confirmed that SAP Programs are designed in such a way that the Re-organization program for such horizons atleast are triggered to run every month. this is not a confirm point But i read it somewhere.