The big things to remember when looking at that chart are:
- Directly tested kanji are a very small part of JLPT
- There’s no “official” JLPT list of either kanji or vocab any more – the test might contain anything the test setters consider reasonable for somebody at N2 level to know
- Knowing a kanji alone is not all that helpful – you need to know the meanings of the words it’s in
- If you do encounter an unknown kanji or word in a reading question on the test there’s a fair chance you can make a good guess at the meaning anyway
For me, these things add up to it being a clearly better time tradeoff to study all the other things the test requires (grammar, vocab, reading proficiency) rather than pushing your WK level up that chart from a theoretical 60 or 70% coverage to even 80 or 90%, let alone 100%.
I also tend to feel that pure vocab study is in a similar “useful but don’t focus too hard on it” bucket – there are lots of words that might be on the test, and most of them won’t turn up in the specific exam you take. There are a lot fewer grammar patterns, so they’re much more likely to turn up on your exam. So grammar and reading and listening practice tend to have better expected payoff than kanji or vocab study
(But it’s also good to do some practice tests to see where your personal areas you need to work on are.)