As a linguist, I quibble with some bits of the article. It is not true to say that if Lewis Carroll had used "they" when referring to Alice, he would "merely have been following" Dickens and Austen. Dickens, Austen and other writers used singular "they" to refer to a person of unknown gender (an indeterminate individual, as in Austen's "“I would have everybody marry if they could do it properly"). Occasionally, "they" was used where the gender was known but considered irrelevant, as in Shakespeare's "There's not a man I meet but doth salute me as if I were their well-acquainted friend" (though this could also possibly be interpreted as the old use of "man" to refer to a non-gender-specific individual). But all these usages are a different kettle of fish from "Alice was beginning to get very tired of sitting by their sister", which would have seemed very odd to Carroll, Dickens, Austen, and Shakespeare.
Secondly, we are told that "Old English used variations of the same word – “he” and “heo” for “he” and “she”, and “hi” for third-person plurals of all genders. But in day-to-day life these could be confusing." OK, if it was confusing to use a similar word for singular and plural 3rd-person pronouns back then - so confusing that "hi" was replaced with "they", even though the pronouns were only similar and not indentical - why then are we be encouraged to think that using "they" as a singular (to refer to a definite individual, not the traditional indefinite usages) won't be confusing and that anyone who says it is is some kind of bigot?
"In April this year, the US became the 17th country in the world to introduce gender-neutral passports, offering the option of “X” alongside “M” (male) and “F” (female). We live in a globalised world where many languages, such as Finnish, don’t have gendered personal pronouns; there’s no one size that fits all."
This is lumping different points together in the same paragraph. The fact that Finnish lacks gendered personal pronouns has never stopped them from distinguishing M and F. They don't all feel nonbinary. I don't think Finland allows X on passports yet. In fact a Finnish government website states that you must be entered on the national population register as M or F - you cannot be nonbinary: "The Finnish law only recognises two genders: man and woman. In Finland, a gender-neutral person is always entered as a woman in official documents and registers." ( dvv.fi/en/gender-recognised-abroad )