If i recall correctly from pathfinder, if an ability asks for a die number higher or lower than something else that superceeds the nat 1/nat 20 ability to always hit/miss.
This is from pathfinder but would suggest that the Defensive roll would trump the natural 20
Natural 20 and Natural 1: On attack rolls and saving throws, a natural 20 is an automatic success and a natural 1 is an automatic failure. But should I treat them differently than other results when deciding if a roll succeeded or failed by 5 or more, when comparing two opposed attack rolls to see which is a higher result, or other similar situations?
No, unless a specific rule tells you otherwise, treat a natural 20 or natural 1 result on an attack roll or saving throw the same as any other result when comparing the total result to other numbers. For example, if a fighter rolls a natural 1 for a total of 31 against the wizard’s AC of 33, the attack misses by 5 or less and destroys one of the wizard’s mirror images.
I don't know if that ever made it into starfinder but i'll look