You're allowed to Fall Back, because you're allowed to move as if it's your Movement phase. When you're moving in the Movement phase you have several options, including the options to Fall Back or Advance. You can't use Warp Time to move normally even though you're within 1" of enemy models because you're supposed to move as if it's the Movement phase. What does "as if it's the Movement phase" mean if not that you're supposed to follow the rules for moving in the Movement phase?
You can't use Warp Time to Fall Back and then shoot, though, because the rules are clear that it doesn't matter when you Fell Back.
See:
"A unit that Falls Back also cannot shoot later that turn unless it can FLY." (p177)
"You may not pick a unit that Advanced or Fell Back this turn..." (p179)
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