In the proof of Theorem 10.4.1, we showed by an arbitrage argument that the value at time 0 of a payment of backset LIBOR $L(T, T)$ at time $T+\delta$ is $B(0, T+\delta) L(0, T)$. The risk-neutral price of this payment, computed at time zero, is
$$
\widetilde{\mathbb{E}}[D(T+\delta) L(T, T)]
$$
Use the definitions
$$
\begin{aligned}
L(T, T) & =\frac{1-B(T, T+\delta)}{\delta B(T, T+\delta)}, \\
B(0, T+\delta) & =\widetilde{\mathbb{E}}[D(T+\delta)],
\end{aligned}
$$
and the properties of conditional expectations to show that
$$
\widetilde{\mathbb{E}}[D(T+\delta) L(T, T)]=B(0, T+\delta) L(0, T) .
$$