Why no need for a court case, Redlion?
She lied. She refused to apologise or retract the lie.
Prosecuting someone for libel is extremely, prohibitively expensive. As a rule, you have to pay as the case goes on - so even if you win, and costs are awarded against the libellous person, it is still too expensive to actually embark on a legal recourse. That expense deters normal, non-super-rich people from seeking restitution when people with access to a wide dissemination of an untruth use that powerful platform to lie and defame.
As a rule, KH would have had the resources of the Daily Mail behind her, fighting any court case an individual took against her, who would have paid her costs, and then printed a teeny tiny retraction.
This inequality of final outcome (the lie gets a big splash, and wide dissemination, the retraction is barely noticeable, thus the lie tends to pass into people's beliefs) is another thing that puts people off from seeking restitution.
The ultimate outcome is that the powerful, and those who are protected by the powerful, are pretty much able to lie with impunity.
Sadly for KH, she lied on Twitter, not in the Daily Mail. And didn't apologise. And didn't retract. And JM decided to press ahead with that extremely uncomfortable, time-consuming, stressful, expensive business of prosecution.
And so, alas, KH went to trial. And lost. And the costs fell on her.
Legally, she was at fault. I would suggest she was also morally at fault (in not avoiding legal action by apologising).
It's hard to think of any way that this wasn't her fault.
And it's worth emphasising that the financial penalty was one which was objectively set by a court - it wasn't some randomly punitive figure. It was a legal penalty/costs.