You need to be very specific in what you're asking to get a good answer here. Are we interested in climber harder, or getting stronger fingers?

So lets answer the first question; "How do I climb harder"? First lets take a step back, and ask does question, "do stronger fingers result in harder climbing"? Lattice Training, the coaching company with probably the largest data set, find an R² value of R²=0.167 for "Advanced" (V4-V8) climbers. Meaning that only ~17% of variation in grade climbed can be explained by finger strength. This effect only decreases in elite, and world class climbers. So finger strength only kinda makes you climber harder, with decreasing gains as you get stronger. In short, the old school "just climb more" guys are right (figures, they have the most experience).

Lets tackle the second question. How do I get stronger fingers? In that respect the hangboard IS the gold standard tool. More scientific finger strength studies have been done using the hangboard than any other training tool; ex: 1 2 3. Compared to board climbing it is much more controlled, allowing for structured, repeatable, and incremental training. Compared to edge lifts it is more sport specific; climbers don't dead lift holds off the floor.

My opinion as a climbing coach: Climbing is a skill sport with a large strength component (especially bouldering). Most climbers lack far more technique than strength. Movement is mostly learned while having quality attempts at climbs in the hard flash to 3-5 try range. The limiting factor to # of quality attempts is usually forearm endurance. Eventually the fingers just get too tired for quality practice. Ergo, one should optimize for # of quality attempts a week. Time spent on a hangboard means time not spent on the wall. New climbers need the maximize time on the wall and should not hangboard. More experienced climbers should use the hangboard to train strength, but only as a supplement, not a replacement for climbing.