This implies all such causal chains, regardless of their nature, will always be accepted as valid. If that were true, no lawyer would ever lose a case.

It's the same argument that people make when they say, because any word can be claimed to mean anything, any so-called hate speech crime can be used to make any arbitrary speech illegal, simply by labeling any form of speech "hate speech."

It's the 'perfectly spherical cow in a frictionless void' model of society that assumes societies are not made up of humans with brains already aware that people can lie and attempt to game the system, and that no one will ever be willing or able to correct flaws in the system. Even in the case of OP, I doubt the UK could take any arbitrary tweet and sentence someone under the same law.

That said, I think the laws in the UK in this regard are going too far - but a slippery slope implies an irreversible process. These laws exist because the people of the UK want them to. If they wanted otherwise, they could change the laws to reflect that. That isn't a slippery slope.