Lilaena is Mandalore, but the only people who know that are on Onderon. And the people of Onderon don't connect her with Lilaena the terrorist because that isn't news they've paid attention to (I mean, galaxy wide she hasn't made THAT big of a splash. She's just irritated the Imps.)

The Mandalorians of Onderon are for all intents and purposes unknown in the galaxy. Other Mandalorians don't really know they exist, no one else is going to know or care... except for the other people on Onderon who they've been fighting for generations.

Anyway, LD isn't going to go up to the Rebels and be like "hey I've got Mandalorians," that is very much a separate part of her life. So yes, a secret. And she's got lieutenants taking care of the tribe while she's away.

Eventually LD wants to take her Mandos off Onderon, and maybe we'll go to Ambria. What she'd rather do is get an ISD or something, crew it with her guys, and go wreak havok.