How so?

Double spending with 0-conf can already be done through tools like the ones Peter Todd used against coinbase, without RBF. It's not RBF that makes 0-conf problematic: https://np.reddit.com/r/Bitcoin/comments/40ejy8/peter_todd_with_my_doublespendpy_tool_with/cytlhh0

But RBF can be used to bump-up fees in order to get low-fee transactions unstuck.