Is it a spoiler if Hermios already more or less mentioned that
earlier
As for why she didn't steal it earlier, at least based on what's shown:
1. She said she'd rather hide this if possible, probably because the ability to steal someone's skill is going to put her very high on target list once it's found out (since, y'know, there isn't absolute guarantee they'll all be friendly forever)
2. Given her panic when the boss turn to her for that, her own combat stat is probably very low so aggroing the boss earlier would risk someone else having to tank hits for her or her getting taken out before the boss go down.
3. Speculation: The amount of time she can keep the skill is probably limited (either hard limit or duration depends on strength of the stolen skill) so she had to wait until the boss got whittled down a bit first and use it for a final push at the end.