Yes, the very basis of LN is secure forwarding between payment channels. Alice wants to send 1 BTC to Bob. Alice is connected — via a payment channel — to Mall

Yes, the very basis of LN is secure forwarding between payment channels. Alice wants to send 1 BTC to Bob. Alice is connected — via a payment channel — to Mallory, and Mallory is connected to Bob. To do this Alice can send 1 BTC to Mallory, and then Mallory can send 1 BTC to Bob. This requires 1 BTC to be locked in both payment channels. The only thing LN guarantees is that the transfer is atomic, such that Mallory cannot refuse to forward the 1 BTC (received from Alice) to Bob.