A street snack that has baffled botanists
This looks like a giant variety of 'Pachyrhizus erosus' or Jicama, which is also a street snack widely sold in Mexico. I'v never seen ones as large as the one in the article, but the watery texture and how it's condimented (lemon, sugar, salt, chili powder) is exactly like Jicama. Seems wikipedia has a reference to ones as large as 2 meters and 20 kg https://en.wikipedia.org/wiki/Pachyrhizus_erosus so it may be like those tree trunk sizes in the article.It's not Agave. Agave roots are very fibery and dry, there's almost no moisture in them and you wouldn't be able to cut such thin slices like the article points out, agave roots are mashed up to start the Tequila process, here are some pics http://www.todomezcal.com/Elaboracion/molienda.htmlSource: I've cut open both Agave and Jicama plants, I grew up in Mexico. You can find Jicama/(fruit) street vendors in most Mexican cities main streets or outside Mexico a mexican market is also a safe bet. They have rough peel,  like a potato but rougher, and they're about 90% water.