Newpipe is accessing the videos straight from the backend. There is no chance to splice any ads into, unless youtube were to modify the source material, and that’s highly inconvenient. And then we’d just use sponsorblock anyway.
They tried to change the code to access the raw video material many times over, but unless they encrypt it and enforce decryption via keys uniquely embedded in the official youtube app while somehow finding a way to prevent a disassembly to use their keys in unofficial apps, I don’t see that happening.