Most memory bugs in Chromium are in V8, either entirely in the JIT or at the boundary with C++. Rust wouldn't help here because the borrow checker can't see through these boundaries, and it's precisely this opacity where the developers also lose track of things.
Which isn't to say Rust wouldn't have caught many of the other memory safety issues, but 75% is horribly misleading.
In Chromium which is the condom of the internet, already millions of developer hours of the most highly paid developers have spent their days trying to make it water tight. This is not your average software project.
Which isn't to say Rust wouldn't have caught many of the other memory safety issues, but 75% is horribly misleading.