The chapter implies the seraphim are young so they’re untested not that they’ll grow stronger.
And you’re using trained gifts like strength; but how does one train how hard their skin is? You can’t.
Why do you think not being tested and being young are related? It's because they can't risk testing a young Lunarian against lasers, which literally translates to their defense getting better when they're older making them ready to get tested against lasers.
Again, this chapter legit implied it gets better with age. Lunarian defense is a racial trait plus a fire-based trick. So if they use fire to somehow fuel their defense mechanism and speed, bettering their fire prowess could help in improving it. People can train to improve their durability although there are limits based on your race. Vergo for instance is built like a tank. Jimbo is tangier than your average FM fodder. Kaido should be more or less than the same in relation to the other Oni. Being gifted with Oden's physical prowess helped Momo ignore attacks that would hurt a regular human, etc.