Backed by liquidity or by assets? I thought the article said that 90% of the funds were liabilities
If exchanges are using fractional reserves of any kind or amount, they have already committed fraud.
It means they have 10% of their deposits held as cash. The remaining 90+% is in liquid marketable securities (usually mortgages and other loans which are freely traded if they need the liquidity)
That is kind of the whole point why we have central bank intermediation and why every country has adopted it.