Everything they have shown is new history. Actually, outside of Mando, all that has been shown of boba is pre-New Hope. Mostly him as a kid.
I sat down with my GF and she gave me a timeline as to when all this is taken place compared to all the Star Wars movies. She showed me this....
9 ABY - A lone gunfighter – Din Djarin – makes his way through the outer reaches of the galaxy where he stumbles upon a young-ish green creature. The warrior goes on the run, hoping to keep the creature away from harm, and ends up adopting the young creature as his own.
Din hopes to reunite the creature with the Jedi. He eventually meets Ahsoka Tano, who is on the hunt for General
Thrawn. She tells Din to head to Tython and use a special stone to call to any Jedi who may be out there. Ahsoka also reveals the creature's real name: Grogu.
Grogu calls to the Jedi but is then captured by the evil Moff Gideon, who holds the Darksaber blade. Din gets the help of previously-thought-dead Boba Fett and the former heiress of Mandalore, Bo-Katan, to beat Moff Gideon. They almost succeed, when a battalion of Dark Trooper droids arrive. Luckily, Grogu's call to the Jedi is heard, and Luke Skywalker saves the day. Skywalker then takes Grogu away to be trained as a Jedi. (The Mandalorian)
9 ABY – Former bounty hunter Boba Fett arrives on Tatooine with Fennec Shand to take over the planet's criminal underworld. Their plans are thwarted by Tatooine's mayor, along with the Hutt twins, and the Pyke Syndicate. (The Book of Boba Fett)
What I meant by back history, compared to where the Star Wars movies are now some 35 years ABY, The Book Boba Fett is 25 years back in history. However you're right it's all new history that we haven't seen or learned about.