Imaging writing a 100+ page document and doing Menu > Syntax Control > Adjectives and seeing all the places where you use unnecessary adjectives. Or really long, run-on sentences. Or extra semicolons. These may be correct grammatically but they might lower readability so you want to minimize their use.
I could be wrong but I think this is what they meant by Syntax Control - ability to detect syntax use in English and not highlight/fading text color. If it is the former, I can absolutely see why it took four years to create.
It's exactly this: you select "verbs" and the verbs are highlighted. You select "nouns" and the nouns are. Same for adjectives, adverbs, prepositions (as, from, in) and conjuctions (and, or).
Nothing earth shattering, nothing you can't do with simply tagging words, nothing marchine-learning or AI about it.
Here's an example:
P.S Damn, it's even worse than that. All the hard work -- which was not that hard itself -- was already made by Apple in their CoreText API. According to the Verge:
"the heavy lifting is already baked into Apple’s developer platform. Since iOS 5 and OS X 10.7, Apple has provided a class called NSLinguisticTagger that segments natural-language text and labels the text with various bits of information, including parts of speech".
Programming can be a verb and a noun, and it just happens they have it tagged as a verb -- context and syntax be damned.
(Or actually, it's Apple's API that does the tagging. They just take advantage of it, and do the oh-so-difficult work of highligting the tagged words)
Unlike, say, Word's "syntax correct" feature, "Syntax Control" has very little to do with recognising syntax.