The "CDC" doesn't state explicitly which smallpox vaccine they refer to here (newer ACAM200 or Jynneos?), but it's most likely the two newest vaccines. According to the article, at least 85% effectiveness (assuming newest vaccines). 
Recent scientific paper:
It is estimated that over 70% of the world’s population is no longer protected against smallpox, and through cross-immunity, to closely related orthopox viruses such as monkeypox. Monkeypox is now a re-emerging disease.https://www.sciencedirect.com/science/article/pii/S0264410X2030579X (Volume 38, Issue 33, 14 July 2020, Pages 5077-5081)
Being a virus, it's susceptible to mutating as well. From your linked article:
""It does seem to be spreading, at least from the information that we have so far, through human to human contact," she told CBC News.
"Which means that it's either that it's a more transmissible variant of monkeypox than any that we've seen before between humans, or there are behaviours that are associated with increased transmission."
Don't worry though, it's ramping up just in time for the mid-terms.
With covid largely flaming out with most of the population, they need the next virus to be even scarier so they can go forward with another round of lockdowns and mail in ballot only elections.