What do we mean by MHC molecule diversity? Does each human have a variety of MHC molecule isoforms?
You're right that both polygeny and polymorphism contribute to the diversity of a given gene in the population correctly described what they mean individually. 

However, you missed that the polygeny of MHC means that the human genome has 3 different loci/genes (called MHC-I A,B &amp; C) in the haploid genome, meaning that any given human as actually 6 copies of the gene (i.e. 3 copies from each parent). All of these copies perform the same function of presenting peptides to CD8+ cells, but will have slightly different binding preferences for the peptides that they load. This is where the MHC diversity in a single human comes from.

Additionally each of the 3 MHC-I loci/isoforms is polymorphic, meaning that for each of them multiple alleles/variants exist in the human population. Since the number of these variants is pretty big (hundreds to thousands), most humans have different alleles on their paternal &amp; maternal chromosomes and end up with 6 unique MHC-I genes.