As explained in the course notes, pump, chwech, cant all drop their last letters in front of nouns. The mistakes in the course phrases will be corrected when we can, but in the meantime the correct versions are accepted.
Why is pum also five in this context? Is it absolutely interchangeable in any context?
See the course notes on 'Numbers'. In front of a noun, pum is correct. At the moment we cannot easily get rid if the incorrect pump version.