It's not just doing false positives, the system is also doing false negatives.
I shared Masonry with theory the other day, and it instantaneously decided that theory had no castle cards to consider melding. So I knew he had no castle cards in hand.
I then used this information to destroy him.
I really hate to be a complainer, but the constancy of the timing of the pause still gives away info unless people start meta-ing it by timing their choices to be as close to the auto-select as possible. And then the approach still is missing some cases like Masonry so that's not good.
The functionality that we need is the "Umbrella with no cards to score" bug. That would actually be perfect for nondecisions. "Check up to 0 cards to meld with Masonry, theory". Sure thing boss. Timing is actually based on the human behavior.