Another question is the timing for these Great Works. The High Republic lasted for about 400 years, from 500 to 100 BBY. Lina Soh came into power in 234 BBY, so about 100 years before The Acolyte in 132 BBY, which showed the beginning of the end for the High Republic. She started all the Great Works, with Starlight Beacon being the first, having a formal dedication in 232 BBY. Now, while we know At Attin is taking part in one of these Great Works, was it and those other planets already hidden before that for other reasons, or were they all part of the Great Works and hidden post-232 BBY?
After reading up in Starlight Beacon, it was destroyed by the Nihil pirates only two years later, so I am thinking maybe these planets were not hidden until after that, to try and prevent their destruction also? Though as episode 3 tells us, only At Attin survived, so whatever methods were used did not work very well. So At Attin could have been in isolation for at little as 250 years, or as much as 400, if the hiding pre-dates Lina Soh and her Great Works.