The "zero reserve requirement" means that banks are now required to keep $0 in their account with the Federal Reserve Bank for every $100 of deposits they take in--a literal pile of cash in their bank vault would not count a dime. That is the only thing that qualifies as reserves for that requirement, and hopefully you understand that that is by no means an accurate reflection of any lay person definition's of "reserve".

Instead, modern financial regulations use capital requirements instead of reserve requirements. In essence, banks need to keep a buffer of their own cash--this isn't part of the assets/liabilities calculation--that can be raided if assets prove to be undervalued, and the amount that is needed depends on the risk of the assets. Something safe like cash requires no equity to be covered, while something like a mortgage might require about 3.5% of the par value in equity, and something like cryptocurrency 100%.

Note that you can also judge banks by how much their assets exceed liabilities. A typical bank is seen as perilously close to insolvent if their assets are "only" 107% of liabilities. By this metric, I will point out that Tether's assets are about 101% of liabilities the last I checked--and that's assuming that Tether's statement of its assets are in fact accurate, which given their history of lying, is not necessarily a safe assumption.