"Liquid" in the context of finance and economics generally means things related to cash. As Merriam Webster defines it:
consisting of or capable of ready conversion into cash
liquid assets
The entire series where the sentence you are asking about comes from is about money and currency. Liquidity is a business concept
In business, economics or investment, market liquidity is a market's feature whereby an individual or firm can quickly purchase or sell an asset without causing a drastic change in the asset's price... Money, or cash, is the most liquid asset, because it can be "sold" for goods and services instantly with no loss of value.
As StoneyB points out: cash itself (the dollar) is the lie. It has no actual value--but it is a "lie" which passes as "truth" because it is liquid-- "it can be 'sold' for goods and services instantly with no loss of value."