1. "How certain are you that Noah's Ark didn't exist?"
I am 100% certain that Noah's Ark didn't exist because it is biologically impossible for the story to be anything near true. You can't fit "2 of every animal" on an "Ark". You can't fit enough seeds, cuttings, etc. on an "Ark" to make the earth green again. And if you were to "Flood the earth for 40 days and 40 nights", that would make all the water on earth brackish and that would kill a vast majority of aquatic life, including both animals and plants.
Genetically speaking, we know so much about so many species that we know that this story is nonsense... Rice DNA alone is 50X more complex than Human DNA and the various breeds of rice show that it's far older than Humans and we can track the changes as it grows across the landscape... RICE proves that the story is nonsense.
2. "Dr Irving Finkel seems to be 107% certain it didn't exist."
You can't be more than 100% sure of anything.
3. I've read a book that says that Humans are aliens that were seeded here by aliens on Planet-X as a slave species to mine gold and that all world leaders are really Reptile Aliens. Anyone can publish a book on anything.
If he is sure about his views, he'll be able to provide evidence to support them and so far the "Evidence" to support the notion of Noah's Ark is easily debunked nonsense. If he doens't believe in the myth, it's not hard to debunk it.