Personally, I think the reason they used it is because most of us have heard of it - you lucky thing, you poor thing.
And someone in their marketing team thought it was the best choice of word to describe a teenage girl buying a bra, especially by adding the word fearless.
To me, arriving at 'fearless thing'. would indicate one hell of a journey in trying to find the correct wording.
Strong girl, young woman, powerful lass. They all sound a bit shit. But that's because buying a first bra is not an indication of your bloody personality. So it sounds a bit wank to attribute having breasts with being brave.
And I bet you any money there was a discussion around gendered language, too. And whether to be specific or not.
And maybe, possibly, the un-gendered nature of the word thing covered that base too.
it looks to me like the product of a massively tortured series of meetings and suggestions, which is why it has come out bearing no actual resemblance to teenage girls buying bras. And why everyone has an opinion about what it really means.