This whole situation seems totally bizarre to me unless the defamation laws in the US and/or France are totally different in the UK.
In the UK the burden of proof in defamation cases is reversed to that the claim is presumed to be false, and the burden is on the person making the claim (i.e. the defendant in this case Owens) to prove that it is true. Under UK law there is no onus on Macron to prove that the claim is false. However the claimant does need to demonstrate that the defamatory statement has caused harm.
And this is surely as it should be - otherwise anyone could make defamatory claims with impunity in cases where it is impossible to prove them to be false. Even though this is a case where it is possible to demonstrate the truth one way or the other, why on earth should the Macrons be expected to prove that she is a woman - the onus must surely be on Owens to prove that she is not.
But perhaps the law in the US/France is different, which would be nuts, meaning that someone can make a defamatory statement and have no requirement to prove that it is true if challenged in law.