The lockdown still reduces the risk to essential workers in three ways: 1) Much of our normal daily contacts is outside of work, that part of a lockdown affect

The lockdown still reduces the risk to essential workers in three ways: 1) Much of our normal daily contacts is outside of work, that part of a lockdown affects everyone; 2) The essential workers encounter less people because others are out of the streets; 3) If the general spread is lower and slower, then there's less chance for everyone, including the essential workers to get infected. It's just as with vaccination - if we get a sufficient number of people vaccinated, then everyone is protected because the disease doesn't spread; and if everyone non-essential is on a sufficiently strict lockdown (instead of classifying anything remotely related as essential) then everyone including the essential workers is protected as the disease dies down instead of infecting everyone who's susceptible.