To be honest when you read the summary conclusion it is not surprising it is being misreported
On page 2 “ During the initial 10 days of the third COVID-19 lockdown in England in January 2021, prevalence of SARS-CoV-2 was very high with no evidence of decline.”
But on page 3 in the results section “Since there was a gap of over one month from the end of round 7b on 3rd December 2020 to beginning of round 8a on 6th January 2021 we may have missed a peak in prevalence during the intervening period”
Reported rates from PCR testing increased from 150/100,000 on 3 December to to 600/100,000 on 6 January ie a fourfold increase so not surprising that they found an increase in that time