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.
I'm more inclined to believe the experts in this instance.