When Frieren brought it up the first time, it came with the implication that Fern is naturally even better at concealing her mana than Frieren herself.

Now we know why she thought it was such a useful ability to have. Love the absolute imperious way Frieren and Fern glare down at their enemies while floating. The demon guy is right, it is purposefully demeaning to demons, as is the way that Frieren has seeded human magic systems with demon magic. They get unsettled that their "life's work" gets surpassed and amplified by 'weak' humanity.

Stark is no slouch either, he is really strong and trained exceptionally well to react in battle appropriately. I thought it was funny that he though Fern could jump up and grab his hand from that height. Plus, he understands the value of a tactical retreat.