The models that were placed but had forfeit their movement have to forfeit their combat action per this section “models suffering an effect that did not allow them to advance are placed normally but must forfeit their combat action…” Second paragraph after the unit movement header. This means they would get placed but not be able to make attacks
Except the line is “Models suffering an effect that DOES not allow them to advance …”
The section on forfeiting normal movement states “A model that forfeits its Normal Movement can still advance as the result of a special rule, such as overtake, side step, reposition, or sprint”
Therefore troopers who sacrificed movement are not currently suffering an effect that does not allow them to advance. They simply forfeited the opportunity to so with their normal movement but as you already quoted above, this does not prevent their placement or impact their combat action.
An example of an effect that DOES prevent them from using their combat action in this scenario is shadowbind “A model suffering Shadow Bind suffers -3 DEF and cannot advance for one round”
The Unit Movement rules also state that “Unit movement only applies to Normal Movement and does not apply when models move for any other reason, such as when moving as a result of the Reposition special rule.” I am skeptical that “suffering an effect that does not allow them to advance” was intended to apply only to Shadow Bind-type effects.
Of course, I would welcome official clarification.
A model that cannot Advance is one that can’t under any condition. While Knocked Down, Stationary, Shadow Bind, etc.
Forfeiting your movement is how you spent it. But you still could Advance by another rule.