Did you read this part of the article or only the headline...?
"While omicron seems milder overall, the UKHSA has found it is not necessarily mild enough to avoid large numbers of hospitalisations. The experts have found evidence that for those who do become severely ill, there is still a high chance of hospitalisation and death.
Given that the transmissibility of omicron is very high, there is the chance that even though it is milder, infections could soar to the point that large numbers end up in hospital."