"so we use an algorithm that relies only on the number of distinct values for both relations together with their null fractions:"
maybe adds a parenthetical note:
"so we use an algorithm that relies only on the number of distinct values (the row count estimate for the whole table, not the -1 in the column statistics) for both relations together with their null fractions:"
Just note I haven't tried to absorb that whole page, let alone the implementation, and am not all that familiar with this part of PostgreSQL. Its seems right, though, in isolation.