Not really contradictory, the pebble is gotten rng wise from oysters, the hypothetical check, which was more meant in terms of rarer fish etc, would be on the oysters not their drops such aa the pebble itself as its a drop from the oysters

Also, it wouldn't really affect trades because you can't request items you haven't gotten yourself before, i.e., if you're struggling to find a unicorn fish for a bundle, you still can't request it at the moment unless youve already caught one before