Yes, that's true, but there is a working range where some hardness will yield longevity. The chipout that I'm talking about isn't a matter of chips breaking off in an undesirable way (like would occur on an overhard tool), it's a punch out of a notch from silica particles.
It happens at any hardness, vs. overhard tools that will release the small chips even in good material.
The hardness of the iron that i'm using in rosewood here is not that high. its probably about 60. what I haven't compared is the same O1 iron that i used in testing, which is a bit above that, but shows very good toughness in particles that are probably similar composition (in the maple), but concentrated in a spot.
The overall issue here, away from the toughness issue (that's one where chart toughness, at least in my experience, doesn't always translate to performance in wood in every case), is whether or not the longevity relationship holds in less than ideal materials that don't immediately demolish an edge. I thicknessed and smoothed the guitar and fingerboard blanks without any real issue other than noticing that in this particular blank, the appearance of small lines occurred pretty quickly. By the time the iron saw the stone again there were a lot of small lines.
I will probably write no matter what in my report that searching for a tougher iron to plane bad material really isn't a good plan. I went down that road first, and found a change in practice (rather than a change in the plane iron) far more beneficial.