I don't think you'd necessarily have deniability. We have early warning radars and satellite networks now capable of identifying an ICBM missile launch in the boost phase. Even with only ground-based sensor detecting the missile in the midcourse, it is a ballistic missile, which means the missile follows a predictable trajectory. This can be used to fairly precisely determine what it is aiming at, but also could be used to trace the missile back to a launch site.