Rui had memorized tons of general information regarding the Beast Domain, and he had run into countless recorded phenomena that humanity had failed to comprehend. Despite occupying a minority of the Panama Continent, it was far and away the most mysterious chunk filled with all kinds of supernatural phenomena.
The Beast Convergence Hypothesis, lost cities, primordial seed, the Warp, and many other phenomena took more central notice because they were observable mysteries, while the Garden of Salvation was entirely centered around second-hand information.
That was why Rui had simply logged the information into the enormous metropolis that was his Mind Palace and forgot about it. He actually didn't even organically recall the name through natural memory; he had to search for the information in his Mind Palace in the part that stored information about locations. After searching for anything that contained the word 'garden,' only then did he retrospectively remember it.