This was a unilaterial declaration by the UN and the CDC - until more cases cropped up in Africa. There is no way to prove smallpox has been eradicated until we can examine everyone in every geographical location over their entire lifetimes.
It's a good bet that smallpox still exisits in many areas around the world. Most likely remote locations in Africa and Asia as well as in the slums of cities in those areas.
It's impossible to prove a negative, i.e., that there is no smallpox, but in this case we're about as sure as we can be about anything.
We've had far more sightings of Bigfoot in the last 25 years than we've had of smallpox.