Quite possibly, it's showing up as mild because it's re-infecting people who have already recovered from COVID and people who are vaccinated. Just read a paper that shows there is a large spike in "reinfections" in SA. But if that reinfection is little more than a cold, it's not really a big deal. Just speculation on my part, but it might appear more transmissible because nobody is fully immune. Here's the paper.
What about those who are unvaccinated? I guess we’ll find out soon.
From the study you linked:
“ Conclusion: Population-level evidence suggests that the Omicron variant is associated with substantial ability to evade immunity from prior infection. In contrast, there is no population-wide epidemiological evidence of immune escape associated with the Beta or Delta variants”
If it evades immunity from prior infection, I expect that it will also substantially evade immunity from vaccination as well.
It also may not bode well for the immunity gained from Omicron infections, being strongly effective against other strains of COVID.