I get this.

> ... in 'full fat' English ...

English is a bastard of a language and getting messier every day as new nations adopt it is their standard language.

Setting the bar where it is well written and unambiguously understandable is IMHO completely fine for a 15$ product.

Having a text spell checked to comply with contemporary Oxford English is likely not the goal of this product.

> English is a bastard of a language and getting messier every day as new nations adopt it is their standard language.

I disagree strenuously with this idea, because it suggests that there is one 'big' English in which anything goes. A better idea is the one of the register[0]: there are many Englishes, many sets of rules. Different rules are used in different regions, by different groups of people, and have different connotations (e.g., the King James Bible was intentionally written in a form of English that was considered archaic at the time because that would make it sound more grandiose).

If I were to use this tool, I'd be using it to ensure that whatever I'm writing is well-received by my intended audience. Because English usage is so varied, I would need to be able to control the register that it uses to ensure that the output is suitable. The fact that the product website doesn't even mention a list of supported languages, let alone supported dialects and registers within those languages, has a very everyone can see what a horse is kind of feeling[1].

[0] https://en.wikipedia.org/wiki/Register_(sociolinguistics)

[1] https://en.wikipedia.org/wiki/Nowe_Ateny

You still need to adapt it to where you are though, people expect this because it causes misunderstandings. If I as a British person go to the US, I know that I can't ask people to go and buy some booze from the off-license and when finished ask them to put their aluminium can in the bin ready for the rubbish lorry while wearing their jumper because that sounds anachronistic.

> contemporary Oxford English

If you chose Oxford because of the Oxford English Dictionary, note that it's not regular en-gb, it's en-gb-oxendict. "the OED often favo[u]rs "-ize" (and its derivatives) over "-ise" for words derived from Greek roots, and may also include historical or less common usages."