I think it's imprecise to say that opening knowledge is unnecessary. What is unnecessary is opening
theory, or more specifically, rote memorisation of opening lines.
This is different from opening understanding. Understanding the importance of tempo, development, controlling the centre, the different pawn structures, middle games and endgames that result from different openings, the plans and motifs typical in various opening complexes. Any late beginner to intermediate player needs to pick and study an opening. The problem is that instead of studying the opening, players try to memorise lines without improving their understanding of the resulting middlegames, and the plans they should be playing for. Then, when their opponent diverges from the main lines(which in my experience happens in 99% of games between players below 2000, because it's very rare that both players have memorised the same long line), they don't know what to do.
I'm a 1900 FIDE player, I have an opening repertoire of sorts. For instance I play the modern benoni with black. An extremely theoretical opening, and yet I have only a small handful of longer lines actually memorised, because they're simply too complex for me to figure out over the board(e.g the b5 lines against Bd3 h3 Nf3 setups). But what I have studied extensively is the strategic landscape of the benoni, games by strong players in the opening, etc. And I have years of experience playing the opening. I know what kind of exchanges typically favour me, or my opponent, what pawn breaks each player should be trying for. And all of that knowledge is crucial for me to get anything out of the opening. I have beaten players tactically much stronger than me in this opening simply because my understanding of this specific opening was better than theirs.
Tactical ability is obviously important, but it's definitely not everything.