The article does not set out any information to back up its claim. Indeed, the article does not even mention the Royal Marriages Act, so the author may not even know of its existence.
The Royal Marriages Act was clear: Should any descendant of King George II (excepting the descendants of princesses who married into foreign families) marry without complying with the provisions of the RMA (by either obtaining the consent of the British sovereign or giving notice to the Privy Council and waiting for 12 months), the marriage would be null and void (under British law). That is the reason why the son of Prince Augustus Frederick Duke of Sussex, for example, was regarded as illegitimate and not permitted to inherit his father's peerages.
King Carl XVI Gustaf's mother Princess Sibylla was the daughter of Duke Carl Eduard of Saxe-Coburg and Gotha, who was the son of Prince Leopold, Duke of Albany, who was the son of Queen Victoria of Great Britain and Ireland. As Princess Sibylla was a male-line descendant of a British monarch, she was not exempt from the Royal Marriages Act. When she married without the consent of the British king
, her marriage was null and void and her children were illegitimate in the law of the United Kingdom.