Admissions to ITU will be biased by circumstances in spring vs September. For example, in spring only the sickest and younger patients were given an ITU because of fear of more patients coming in the next few days. In September, the threshold for admission is likely lower for patients who are at higher risk e.g. high BMI, male, Asian or Black ethnicity.
Despite this bias the proportion of patients who were of black ethnicity in spring was 16% compared to only 7% in September.
The length of stay is much closer to the background rate for ITU admissions. Again there may be a bias with patients being kept longer because COVID patients in spring could deteriorate suddenly and unexpectedly.
The data on age, ethnicity etc is hard to interpret without knowing the control distribution for average ITU admissions.