Because of Nalan and Navi's fallout with Continent Y's higher ups, Nalan never truly received the latest intelligence from them.
Why should they share important information with these people?
The people from Continent Y were unwilling.
And Nalan himself didn't care enough to try to find out about the other returnees because he never thought that it would be necessary.
They were not going to meet anyway.
The only time when he tried to take a look was when he and Navi were staying on the boat for a while and watched the other returnees who came back from the other world. At that time, he and Navi were just thinking of taking advantage of them if can and forget it if they couldn't.
None of them have that much expectation in the first place.
Another reason to go into the sea was simply to avoid being chased by countless magic beasts all day long.
