The Thrawn fandoms are definitely completely different! Long story short: the Thrawn Trilogy (1991-1993) was a popular part of the old Star Wars Expanded Universe. When they created the Star Wars sequel trilogy, the Star Wars EU was rebranded as Star Wars Legends (and no longer considered canonical). But since Thrawn had been such a popular character, they later brought him into the new canon and so there is a new series (which started in 2017). I haven't had a chance to read the new series yet, but I believe there is very little overlap in characters other than Thrawn, and of course they're set in entirely different Star Wars continuities now that Legends isn't canonical (though it will always be canon in my heart...)