How is marginal benefit measured?
The reason why marginal benefit is measured in cans of soda is that this economy only has two goods: pizza and soda. So instead of using money we may as well use soda. Alternatively, in the absence of money this economy is an exchange economy, and the only way to pay for pizza in that case is with soda.

When you move beyond two goods willingness to pay is often measured in money value. However, measuring willingness to pay in money value is just a convenient way of saying: a bundle of "everything else that is available".

In a two good economy,given the current amount of pizza that you have, an additional slice of pizza may thus be worth 5 cans of soda to you. In a multiple good economy it may be say 5\$ where the 5\$ stands for "all the other (combinations of) goods you could get for 5\$.