If you don't feel that quote clarifies feel free to read the rest of the section on monetary policy instruments.
In other words, if they target the interest rate they can't control the monetary base, so, Central Banks don't control the quantity of money in the economy.
If they choose to target the quantity of money in the economy, the tool that they have to use is the interest rate. So, they would lost control over the interest rate.
Modern central banks target the interest rate: they sacrifice control over the monetary base. The central bank don't decide the quantity of money in the economy, they decide the interest rate.
The implication is that they have to cover any necessary reserves by the private banks if they want to keep the interest rate in its target. So, yes, private banks create money when they lend and destroy money when the money is returned. And that's OK because that means that the quantity of money adapts to the activity of the economy.
This is correct. (The Fed has other tools beside open market operations, however, including directly manipulating rates on reserves and the discount rate.) But monetary base != money supply. The Fed manipulates rates to effect changes on the money supply.
"The central bank influences interest rates by expanding or contracting the monetary base, which consists of currency in circulation and banks' reserves on deposit at the central bank"
https://en.m.wikipedia.org/wiki/Monetary_policy