Question for those more knowledgeable: From the abstract it doesn't seem like they compared this group with other people in the ICU - could the lack of sun expo

Question for those more knowledgeable: From the abstract it doesn't seem like they compared this group with other people in the ICU - could the lack of sun exposure for that population play a role? ICU patients with COVID-19 have likely gone from sheltering in place at home to experiencing symptoms that make them unlikely to venture outside to the hospital where they can't leave, all of which means less exposure to the sun. Is a few weeks of staying indoors enough to cause this?