The state transition diagram of BIP 8 is here.
The specific change you refer to was made in this BIP PR by AJ Towns. Once we move into the LOCKED_IN phase of BIP 8 the soft fork is definitely going to activate at some point and so any requirement to signal is unnecessary. The MUST_SIGNAL phase makes it clear to the network that signaling is being enforced by the network. In the LOCKED_IN phase (which only begins after a difficulty adjustment period, 2016 blocks, where the threshold of signaling has been met) it is clear to the network that the soft fork rules will be enforced in future.