No is claiming that Chisel code is always bug free. Just like no is claiming Haskell code is bug free. However the point they are making is that these languages

No is claiming that Chisel code is always bug free. Just like no is claiming Haskell code is bug free. However the point they are making is that these languages are way better in statically catching a whole host of bugs. They also allow certain properties to be expressed in the type system which is essentially having a proof that that invariant holds now matter what.