A longer cable means that there will be greater electrical resistance, meaning that more power is lost in heating the cable.
The statement you refer to seems to be implying that using a long cable won't leave enough power to charge the iPad (and/or that the cable will get excessively hot). Unless that supplier is using exceptionally poor quality (very thin) cables, that seems ridiculously over-cautious because Apple (who make iPads and ought to know a thing or two about them!) will happily sell you 2m and 3m charging cables [if you're prepared to pay their extortionate prices!]
http://www.apple.com/uk/shop/ipad/ipad-accessories/power-cables