Reserve cards and winning
There is nothing in the rules that prevent you from winning based on certain conditions and getting 15 points just triggers the end of the game. Once the final round is done the player with the most points will win regardless of how many reserve cards they may have.

Rules

Reserve a development card


  To reserve a card, a player simply needs to take a face-up
  development from the middle of the table or (if you’re feeling
  lucky) draw the first card from one of the three decks (level
  ; ; ) without showing it to the other players.
  The reserved cards are kept in hand and cannot be discarded.
  Players may not have more than three reserved cards in hand,
  and the only way to get rid of a card is to buy it (see below).
  Reserving a card is also the only way to get a gold token
  (joker). If there is no gold left, you can still reserve a card, but
  you won’t get any gold.


END OF THE GAME


  When a player reaches 15 prestige points, complete the
  current round so that each player has played the same number
  of turns.
  The player who then has the highest number of prestige points
  is declared the winner (don’t forget to count your nobles).
  In case of a tie, the player who has purchased the fewest
  development cards wins.


As a note this is how the ios app works