Not necessarily true:
In the UK, the burden of proof in a libel case is on the defendant to prove that the allegations are true, but not beyond reasonable doubt. Instead, the defendant must prove the allegations on the balance of probabilities. If the defendant fails to prove the allegations, the court may consider any evidence or media coverage the defendant used to substantiate the allegations as factors that increase the damages.
Here are some other things to consider in a UK libel case:
Truth
The defendant can use truth as a defense, but the truth must be actual. For example, the defendant can't justify a publication by pointing to another press report or allegations in a police arrest warrant.
Peer-reviewed statements
The defendant can use peer-reviewed statements in scientific or academic journals as a defense if the statement relates to a scientific or academic matter.
Other defenses
The defendant may also be able to use other defenses, such as honest opinion, publication on a matter of public interest, or privilege.
What we know to be true beyond reasonable doubt - Lin Yu-ting was banned due to failing a test. There wasn't an attempt to challenge this.
We don't know the exact details but there is enough that it public in terms of the nature of the tests to suggest Lin Yu-Ting is male - noting that the threshold required is lower than the 'beyond reasonable doubt' bar in a criminal case. This has been disclosed by official bodies not just speculation in the press, and the disclosure from Khelif's team about abnormal chromosomal results won't help the cause.
There is a lot of speculation around way, but there a fair argument that it is in the public interest to talk about this and unfairness in women's sport on the basis of sex. This alone is a reasonable defence in the UK.
There are also implications to allowing another government to silence free speech in the UK by trying to intimidate in this way. The UK government would be stupid to stand for it, on principle, because of the precidence it sets for other governments to try and do the same when they hear something they don't like.