Surely the diesel and oil could be removed very easily?

Anyway, this comment on the article you reference is interesting:

"The truth is somewhere in between, if the JCB's were mini diggers and secondhand it would make economic sense to bury the kit and not a lot of space would be lost..."

Also this quote from http://www.itv.com/news/london/2014-06-06/hundreds-of-jcb-di...:

"The diggers used are tiny little machines and they can be lifted out with a block and tackle. There is always an exit strategy for the equipment."