Hate to say it because I try to remain open minded to all points of view, but I just don't believe this. This article and the other web sites that mention it (mostly references to this short article on pro-vegan and pro-vegetarian blogs) give little detail about her diet besides one quote from her, "fruits, vegetables, and nuts. It's all there in the Bible."
This is actually beyond veganism since if this is really all she eats since there aren't even any grains. There is nowhere near enough nutrition in this diet by any standards, and nowhere near enough calories for a younger person unless her diet was primarily nuts. Perhaps what she means is that she eats "a lot" of fruits, vegetables and nuts, but other things as well, kind of like that interview with the NFL player who said he is vegan, except when he eats chicken.
The Bible promotes many foods besides fruits, vegetables, and nuts, there is a "Bible diet" or "Maker's diet", but there is a long list of other foods on it, including grains and meat.
Maybe she has been eating this way for the past 10 years or something (since age 98) but I'd be absolutely shocked if this has been her diet for 90+ years and, sorry to say, won't believe it unless someone comes up with more facts.
I was vegan for about 6 weeks and stopped because I felt like I was going to die. A lot of people try a vegan diet and feel great for a few years and then crash. Many celebs have tried a vegan diet to fight cancer and subsequently died. There is no human culture that has survived exclusively on plants, we have just not evolved to handle it.