The essay lost me when it said that the term "Mary-Sue" is sexist. I've heard it used plenty of times for male characters; the claim that it's overwhelmingly or exclusively used for female characters is a strawman
Statistical evidence suggests that it is, in fact, sexist.
That it is also sometimes applied to male characters does not mean it can't be
disproportionately applied to female characters.
Remember, Luke is a beloved hero, while Rey is frequently derided as a Mary Sue...when she was written
specifically to imitate Luke. The writing is certainly more clumsy in Disney's trilogy, but the core character is fundamentally the same. Presenting literally 100% identical characters, where one is male and the other female, does in fact disproportionately generate negative responses for the female version.
It is, unfortunately, very very common that something can be sexist even when it isn't EXCLUSIVELY anti-women. This is improving with time--in part because writers are finally
actually writing female characters, so we get a greater diversity of them AND writers now have more experience writing them!--but it's slow going.
Besides, one of the greatest problems with "Mary Sue" as a concept is that it has always been far too vague and almost totally useless in terms of writing advice. Every time people try to nail down a clear definition of what "Mary Sue" means, it just produces a new generation of "See, it's
totally not a Mary Sue, I didn't do
any of the things you said were wrong!" Attempts to subvert the basic Sue gave us the Villain Sue (can't be a Mary Sue, they're the
bad guy!), the Canon Sue (can't be a Mary Sue, it's a pre-existing character!), the Jerk Sue (can't be a Mary Sue, they're a butthole, no one likes them!), or the Sympathetic Sue (can't be a Mary Sue, the
universe hates them!) Now that those things have become common-knowledge, we're getting third- or even fourth-generation Sues that try to avoid those flagged symptoms without addressing the underlying
disease.
And that underlying disease is very, very neatly packaged up in the candy-vs-spinach thing. Candy glorifies. Spinach humbles. It is possible to have high-candy characters that are still likeable, even lovable (as noted, Sherlock Holmes and James Bond, particularly the latter; he's cooler than cool, the sexiest super-spy, wealthy and fabulous and highly intelligent and funny and badass and...etc., etc.) Doing so is a risk, however.
Reliably good characters eat their spinach, and only get their candy after the narrative has given the spinach some time to digest.