Actually, if you play with the numbers a tiny bit - you'll find that this is not the case. Not arguing whether or not someone is undergunned on that specific ski for a particular use - but pointing out that relatively tiny changes in width have big impact on surface area. Like by 15 times or so to guess at a "typical" number. Artificially, for the moment, just focusing on surface area - going from a typical 180 midfat to a 170cm 99 waisted ski would add surface area that would be borderline impossible to get via length. For example, those 161cm Shamans I was playing with have more surface area than a 184cm Mantra... Just think about it.
And no, surface area is not everything - but don't fully discount it either...