Why Poland and Norway posed different guarantee problems

Did Britain's inability to save Poland make a guarantee to defend Norway inherently unbelievable?

No. Poland was beyond direct British naval reach, whereas Norway was a maritime theater in which Britain could deploy sea power and Allied forces.

The question assumes that failure to save Poland made a later guarantee to Norway equally empty. Britain had no land frontier with Poland, and the Royal Navy could not reach it directly. Making the Polish guarantee effective required a prepared Anglo-French land offensive from France into Germany, not simply a promise of support. Norway presented a different strategic problem because British sea power and nearby bases could act there. The credibility of a guarantee depends on the forces and routes available to fulfill it.