She though she would be an alternate because she though that Cilić would also accept the ALT spot but he didn't. That's all. People get over it, Hopman Cup is a tournament of nations, it has such a format.
Cilic was never going to accept an ALT spot. If he said yes they would have been 1 of the 8.
Hopman Cup wanted Croatia, and Donna & Cilic were going to be one of the 8 teams. When Cilic said no, Donna still wanted to come as the reserve player.