A Multipolygon is a list of Polygons; just iterate over your Multis and store every item individually. Think of this like nested lists in Python: You need two loops to flatten a 2-level list, it’s the same for your Polygons.
Just don’t mix things up: A Polygon consists of an outer ring and inner rings (Linestrings) which themselves are lists of Points. Don’t dive too deep when iterating.
The linked answer is incorrect; might only be working in his certain case, or that person just mixed up the terminology. The major difference between Multipolygons and Polygons (besides their inner structure) is that a Multipolygon can have multiple outer rings.