I got the impression that Cambridge are worried about their academics, many of whom are the wrong side of 50 and quite a few over 70. With no social distancing it is likely that a few students would die and although more will probably die from accidents and suicides, there is so much focus on Covid that the universities are going to be very cautious as they don't want to be the one with that headline.
There was a good episode of More or Less on R4 yesterday talking about risks for schools and it is difficult to be harshly objective about acceptable levels of death, but that is the rational approach.
A doctor friend said 6 weeks to the antibody test, it would be helpful to know if a significant proportion of the student body have had it.