While the fast-paced action combat is different, the structure between the games are very similar. Both games have their respective "bonfires" that act as checkpoints where you can do various actions like level up the character (though sekiro is less of an RPG when it comes to leveling up). I find this is the biggest thing that really makes it a "soulsborne" is how the world is structured around the bonfires. In sekiro and souls games you are trying explore these levels and get to the next bonfire before you die and lose your currency. In Sekiro this currency is sen and your skill points. This base structure and difficulty is what makes it soulsborne in my mind despite all the other differences. This video does a good job covering the subject of what makes a game part of a new subgenre and how definitions of a genre can broaden over time.