Stack Assault Movement

[Blackcloud6]

I'm going through the Infantry Attacks rules today and reading the assault movement rules, it seems to me that nothing, as written, allows you to move a stack of units into an assault. The movement rules are explicit that movement is done "ONE AT A TIME" but the assault rules only say "units" may move into an assault.

Now I understand that an assault is a "fire" activation and that units can combine to fire. So is it the combining to fire rule that allows one to combine units for the assault? Or is there there something else? Or is it really supposed to be that units must assault on unit at time into and assault hex (which would make assaults really tough to do)?