The theory is that it would be someone that is immunocompromised enough to just hang in there for a long time. It's a numbers game. The more people that get infected, the bigger the chance a rare event like this will happen.
This is one reason why vaccine distribution inequity may come back to bite us.
It's pretty simple. You can estimate the genetic mutation rate from the phylogenetic tree since we've been sampling the distribution of Sars-Cov2 genomes since the original Wuhan strain. If that rate is constant and if the variants all mutate at the same speed that gives you a rough timeline. Omicron has 50 mutations that are not related to other known variants. Delta has 13. We've seen delta for about 1 year right? So it's not a big leap to roughly double that estimate for Omicron.
Immunocompromised subpopulation is so small (at least in my country), especially compared to unvaccinated (abt. 30%), that it should be fairly trivial to quarantine them, don't really see an issue here if we are restricting unvaccinated life based on argument about public health.
This is one reason why vaccine distribution inequity may come back to bite us.