I agree with your interpretation in this case, though "ground caused ball to move" is not actually in the rules. Whether the receiver or interceptor has maintained a "firm grip" with only "slight movement" are the relevant questions.
One relevant passage in the rules is a note in rule 8.3:
http://operations.nfl.com/the-rules/2014-nfl-rulebook/
"Note: If a player has control of the ball,
a slight movement of the ball will not be considered a loss of possession. He must lose control of the ball in order to rule that there has been a loss of possession."
However, the definition of Player Possession (Definition 2.7.1) says:
"Item 1. Player in Possession. A player is in possession when he is inbounds and has a
firm grip and control of the ball with his hands or arms."
These aspects of the rules are unchanged from 2014. The NFL did tweak the catch rules to attempt to "clarify" the "football move" aspect of the rules, but that's another story.
So, how slight is slight? It must very slight, for the receiver/interceptor to still have "
a firm grip and control".
Given there was no rule change for 2015 for this aspect of what constitutes a catch, and given past practice and interpretation of "firm grip" in replay officiating, I see no reason why this was not overruled.