Isomorphic in normal use means corresponding in form[1], so functionally isomorphic means “functions the same as” whereas there are other types of isomorphisms, so for example minerals form groups which are considered isomorphic because their crystal structure is somewhat different but equivalent. They’re not functionally equivalent they are structurally similar.
[1] In mathematics the meaning is somewhat stronger. In maths, two things are isomorphic if they cannot be distinguished in terms of structure in a particular context.
