If you can craft an engaging story and you have the motivation to write, an editor (or a thesaurus) can help with vocabulary.
Actually reading comprehension is about everything _besides_ subject knowledge.
It's about the pure ability to understand something you've read -- e.g. to understand tone, intent, turns of phrase, idioms, writing styles, patterns, argument building, etc -- not about having some particular domain knowledge.
In other words if you read an article about a physics experiment but didn't understand it because you don't know what "spin", "wavefunction", or "interferometer" means in QM, then you don't have a reading comprehension problem. You just have a lack of physics knowledge.
If, inversely, you read an article about something you know very well in real life (e.g. baseball), but miss what the author tries to say, mistake the intent of some metaphor, misunderstand the argumentation, etc, then you do have a reading comprehension problem.
Of course to comprehend something you read you also need to be familiar with the thing it's talking about to. But reading comprehension is about everything else but that part -- it's about being familiar with the written word and the ways people express things in writing in themselves [1].
[1] "Fundamental skills required in efficient reading comprehension are knowing meaning of words, ability to understand meaning of a word from discourse context, ability to follow organization of passage and to identify antecedents and references in it, ability to draw inferences from a passage about its contents, ability to identify the main thought of a passage, ability to answer questions answered in a passage, ability to recognize the literary devices or propositional structures used in a passage and determine its tone, to understand the situational mood (agents, objects, temporal and spatial reference points, casual and intentional inflections, etc.) conveyed for assertions, questioning, commanding, refraining etc. and finally ability to determine writer's purpose, intent and point of view, and draw inferences about the writer"
https://www.education.com/reference/article/prior-knowledge-...
http://www.readingrockets.org/article/building-background-kn...
Here's a few articles on the topic with very relevant reference sections if you'd like to know more.
You don't have to define it thusly, though. Whatever SAT tests test for, this is already the dictionary meaning of the term. The except was from Wikipedia.
Here's a dictionary lemma too: comprehension: "a test to find out how well students understand written or spoken language" (listening/reading comprehension).
It's not argued that domain knowledge doesn't factor tremendously in understanding. Just that this is not a reading comprehension intends to measure (heck, the hint is in the very combination of the words that make up the term).
Of course in order to understand some text in a reading comprehension test, domain knowledge will also help.
But reading comprehension is not about any particular domain -- and tests in that don't constraint themselves to only present texts about a specific subject.
I don't think vocab can be taught. The reason is, vocab is very highly correlated with IQ, which is stable thought life and cannot be boosted. A large vocab requires a strong crystallized memory, which is a part of IQ. Most people when they encounter new words either are unable to infer the meaning and or forget it, but smart people do both.
The fact that IQ and vocabulary are correlated across a population doesn't imply that, and it's clearly not true unless you don't think someone can learn a new language later in life. Or you think that a person can learn 10k new words in a new language, but not an extra few thousand in their existing language?
>Competent writers have a varied vocabulary which does not come across as forced
Yes competent writing requires some minimum vocabulary. It doesn't require a vocabulary sufficient to score near perfect on the SAT. Most writers limit the vocabulary they use to match their target audience, so having a much larger vocabulary than your target audience isn't going to be that useful.
Yet, from Wikipedia (https://en.wikipedia.org/wiki/Intelligence_quotient):
>Raw scores on IQ tests for many populations have been rising at an average rate that scales to three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect.