It has its own loot pool, meaning every gold/titanium crate has a guaranteed chance to drop one.
I'm Pretty sure that's not how crate loot works. Correct me if I'm wrong here, but I think it works like this:
Every item is part of a pool, each pool is rolled for it's drops.
Some item pools have an equal chance for all items, others have different chances for all items.
The item picked from an item pool will be the only item obtained from that pool.
For instance, if you get a Life Crystal drop from the pool, then that means you can't get the Enchanted Sundial or the Hardy Saddle in the same crate.
There are two other rules to crate drops, if a crate were to contain no items then it will be re-rolled.
And a crate can contain only one instance of ores or bars. If you get gold bars from a crate, you cannot get any other ores or bars from that same crate.
There's probably a whole bunch of other bunch of chance mumbo jumbo I don't know, but this is what I do know.
I'm getting off topic here, what I should say is that the Enchanted Sword is not only the rarest gold/titanium crate drop,
but the third rarest drop overall, the rarest if you don't count the Enchanted Sundial's drop rate from pearlwood and mythril crates.
Of my 100 hours or so playing Terraria, I've obtained the Enchanted Sword about four times, all of those from stumbling across it in the caverns by chance.
Of my roughly 5 hours of casual non crate potion fishing for the angler, with 122 quests completed I've obtained about... six or seven gold crates, and like two titanium crates.
Of the ten percent chance of getting a crate without a crate potion, with 100 fishing power the chance to get a gold/titanium crate is...
About 0.6%?
That lines up with my faulty memory at least.
Point is, 2% is better than 0%, but wow, gold crates kinda suck.