I found the ruling by the replay official surprising. So I looked up the rule.
This appears to be the pertinent language, from Rule 2.00:
A CATCH is the act of a fielder in getting secure possession in his hand or glove of
a ball in flight and firmly holding it; providing he does not use his cap, protector, pocket or
any other part of his uniform in getting possession. … In establishing the validity of
the catch, the fielder shall hold the ball long enough to prove that he has complete control
of the ball and that his release of the ball is voluntary and intentional. If the fielder has
made the catch and drops the ball while in the act of making a throw following the catch,
the ball shall be adjudged to have been caught.
So, my guess (and a guess is all it is) is that since Hamilton was not in the act of making a throw, the relevant question was whether his release of the ball was "voluntary and intentional". Since Hamilton's drop was not intentional and voluntary, then a catch was not made.
In these situations the commentators talk about being in the act of transfer, but that isn't mentioned anywhere in the rule. The only issue on a drop seems to be whether the player was in the act of throwing. Since Hamilton was not in the act of throwing and the drop was unintentional, the ensuing decision was that a catch was not made.