Why is the closed gain loop -3dB from the open loop gain rather than some other value?
The text says it's a single pole amplifier circuit and that always means that at the cut-off frequency, due to the transfer function gain equation, the output signal is down to exactly half the power of the input signal. Half the power in decibels is \$10\log_{10}(0.5) =\$ -3.0103 dB.
That's what single pole filters do at their cut-off frequency.