After reading their full guarantee page, it seems that they really don't understand the definition of the word. They don't seem to offer any guarantee, but rather just a list of reasons that they believe their plan is foolproof.
Mostly they argue that their lawyers have covered every angle and a lot of big players use them so naturally everyone will respect the value of their online currency. Nowhere do they address what happens if their company goes belly up, which is the primary thing I'd be worried about with this type of thing.
Ironically they specifically state on their guarantee page
Many first-rate payment systems tried to guarantee their stability but
unfortunately failed to fulfill their obligations given to the users
and contractors. Myopic investment policy and court claims of tax
agencies have frequently adversely affected the work of Electronic
Which, personally, would make me even MORE nervous to use their system.
Their guarantee amounts to, other systems have crashed and burned, but we are smarter than those guys and even hired lawyers to confirm it!