The true benefit of Kevlar is in the FST midrange driver, in which the fibers are biased to produce a driver that varies it's effective radiating surface over frequency, and launches what is essentially a square wavefront. The driver literally pulsates rather than operating as a true piston.
The benefit is improved dispersion and breakup characteristics near the upper crossover frequency. A common problem with midrange drivers, which operate over a relatively wide and important frequency range.
The purported advantage of diamond tweeter diaphragms is the significantly higher resonance frequency; the driver is more linear within the audio range as a result.

IMO, the "hype" is mostly just interesting information for audiophiles, which B&W speakers have traditionally been intended for. Pure music lovers will just appreciate the transparent and natural sound reproduction.
The measurements of the latest $1000 603 indicate outstanding engineering; virtually flat response, very well controlled dispersion, no significant resonances, and very clean decay over the entire audio range.
These measurements would not be out of place attached to a $5000 speaker!

BTW, loudly touted frequency response and power handling specs are the most common form of misleading speaker marketing hype, similar to CPU "speed" in the computer market.
Both are aimed squarely at unsophisticated consumers, and usually have little to do with the acceptability or actual performance of the product.

Kevlar, on the other hand is fundamental to the performance characteristics of the Nautilus series, and is unique AFAIK to B&W.
One can hardly blame them for sharing the details of this innovative driver design with consumers.