Es or er?

Koffer is masculine so Sandy asks if you can describe him. Why does Matthias answer with," Es ist etc." Shouldn't
the answer start with " Er ist etc."?

I always thought so too and never got an explanation that satisfied me. It just seems wrong.  Other examples in the lessons are more consistent in using the correct gender to refer to an inanimate object where we would in English use it.

It seems to me that it is because when referring to the coffee in the nomiinative case, the word for "it" is used rathert than the gender, but with accusative or dative, the gender form is used.  This is just a guess based on observation.