Her reasons for defending her friend is kinda wut lol
"Ryuku-kun, Senketsu is a weapon made to destroy life fibres"
"What, don't call him a weapon, Senketsu is not a weapon!"
"But that's what you've been using him as…"
"Raaaggh, I'm never using him again!"
Lol, like if it's a matter of semantics, just tell him that no he's her aibou so fuck off. There's no need for the stand-off at the end. This whole declaration has no emotional weight behind it especially since you know she's just going to use him again anyway probably after going through some sort of redundant character arc on how he's not a weapon, he's her friend...something which was kinda already the situation anyway.