I think you have that flipped. 20% of code covers 80%. As you try to get to 100% coverage of all possible scenarios the complexity gets in some cases exponentially harder as all the low hanging fruit is picked
So he is exactly right : 80% of the code handles edges cases. You are the one who is flipped.
20% of code covers 80%. 80% of code covers 19%.
but the problem is that you need to properly resolve base cases before starting on edge cases. The fact that base case scenarios are not properly solved hints at major problems in execution.
I like to think of it as an order of magnitude increase for each 9. Ex. If getting to 90% coverage takes 10 hours, getting to 99% will take 100, getting to 99.9% will take 1000, etc.
Not specific to code : that is the Pareto principle.