There are many grills out there better than the weber bbq, and Weber has to make those statements, it's a liability if they speak out differently. Only if they made their own attachment would they approve the modification. As many others have stated they have done it, again doesn't prove it to be safe, rather low risk.
Personally, I have a grill I purchased from Costco last year and I also have a camp chef stove, camp chef sells the quick connect, as well as does Walmart.
If you don't want to chance it, just sell the Weber on Craigslist or something and buy one that supports the adapter
2011 Dodge Ram 2500 Cummins
2012 Starcraft Autumn Ridge 297 BHS