I'd actually disagree pretty strongly with this article - I think that assigning specific marks to spelling and grammar would do little to improve outcomes but end up lowering the overall quality of responses by taking away from the focus of the unit.
The study of English in Stage 6 develops in students an understanding of literary expression and nurtures an appreciation of aesthetic values. It develops skills to enable students to experiment with ideas and expression, to become innovative, active, independent learners, to collaborate and to reflect on their learning
Also, I'd say that the article doesn't take into consideration of the fact that markers do indirectly assess those concerns though the language and clarity of the response, which includes appropriate use of spelling, grammar, and legibility:
Demonstrates skillful control of language and structure appropriate to audience, purpose, context and form
Personally I think the current marking criteria is a pretty good compromise - even if you make lots of basic spelling mistakes but your argument flows and makes sense, you get the marks, whereas if you're using big words incorrectly, writing illegibly, or otherwise not communicating well, you get marks deducted. Considering the fact that exam conditions means writing a (usually 6 page I was told) essay in 45 minutes, spending 5 minutes at the end of an exam to proofread your response is a pretty significant time investment and honestly, having correct spelling/grammar really doesn't add much if the response is still around the same legibility and the marker understands the argument well. Especially considering the paper format of HSC exams, it's difficult and time consuming to be editing a second time as it involves crossing things out, squeezing text into margins, and whatnot which I'd say decreases the legibility of the response later down the track.
One more thing is that I don't think spelling and grammar can be assessed fairly - for example, someone studying 16th century poetry or any of Shakespeare's texts is going to have a much harder time with spelling and grammar than someone studying a contemporary text. Would it really be fair to penalise someone for using the using the modern spelling of "sun" versus the 16th century spelling of "sunne"? Obviously that's a bit of an extreme example but hopefully that highlights some of differences in form between standard and advanced texts which might be unfair (as advanced will generally engage with older texts like Shakespeare which requires greater understanding of context and weird words). And just in general, I don't think someone who's able to properly understand and engage with HSC level texts will need to worry about spelling and grammar.
I guess what I'm really trying to say at this point is that this isn't an HSC problem - and I think this article is fundamentally flawed by positioning it as one. The idea that not marking spelling and grammar in the HSC is the "reason our kids can't spell" implies that we're suppose to be teaching spelling and grammar in HSC when it really should have been nailed down much earlier. The article even supports this to an extent:
University of Technology literacy expert Dr Don Carter added teachers were letting down their students in junior years by not focusing on spelling and physical writing activities...
...The comment comes after a damning review into the NAPLAN test - taken by students in years 3, 5, 7, and 9 - was released in September 2020.
The review found writing has not improved since 2011 and the gap remains significant between male and female students. Almost a quarter of Year 9 boys failed to meet the minimum standards, while 10 per cent of Year 9 girls fell short.
Maybe with a different syllabus and with a different exam format I'd support the assessment of spelling and grammar, but I feel that those are sufficiently covered within the current format of this course.
EDIT: RE: The Cat In The Hat and Joseph41 I think that docking marks for x number of spelling mistakes would be a bit counterintuitive since people who write more or try to use more complex/in depth language would be penalised since that would likely naturally lead to more mistakes under timed conditions. I also feel like it might incentivise people to write less or use simpler language. I guess you could do a % of mistakes in relation to word count but that comes with it's own problems.
Sorry for the massive text dump I didn't realise there were already comments and I couldn't be bothered to rewrite my answer