Well, no, since liquid damage isn't covered by the warranty, there is no actual guarantee. There is a claim (and maybe a third-party certification) of meeting a particular standard, but that's different from a guarantee -- its marketing with no risk of having to pay out money if the performance isn't up to snuff in practice.

A guarantee puts warranty money where your marketing mouth is.

I think he just meant that now the waterproofing is good enough to advertise it as a feature. Making it official, so to speak.