Given universe is 0, then life and everything will be 42 but it's also possible that the unspecified operator between universe and the rest (life and everything) is dependent on parts of the universe equation and might be something else
Note that an absurdist or existentialist who likes not having to perform to a divine meaning may prefer this state of affairs to being told what to do. The author is making humour out of scientists gleefully proving nothing matters. But I think glee at nothing mattering should be the normal reaction. Unfortunately, it seems most neurotypicals would rather be told what to do.
Optimistic take: because they just rearranged it to equal 0 by subtracting something from both sides as is often done in "these quantities are conserved" types of equations in physics!
My favorite is when they discovered the universe has zero energy curvature which means nothing goes in and nothing comes out. Somehow it made more sense that zero could split into thing and antithing and we don't even question it anymore.