You forgot about archeology. The Old Kingdom of Egypt. The cave art that has pre-flood mammoths but no pre-flood dinosaurs. That too is bunk? What about alluvial plains? Fossil fuel? Remains of ancient meteor strikes? Finally, why didn’t the unicorn get on the Ark?
It was one of the species that had been engineered by man. One of many like satyrs minotaurs, and other trans species creatures of old. That is why this world was washed.
The platypus looks to me like one of the very few that endured the flood. It looks like who ever designed it was having a laugh, or using up leftover parts. A poison duck beaver kangaroo bird. Just what kind of environment would it take to favor that kind of mutation? A lab?