Does Alpha Strike target units? If so, it can be countered by Not So Fast or needs to pay Deflect. Also, does damage assignment happen as you play or during finalization?

Yes, Alpha Strike targets units.

According to Rule 355.14.a, if a card specifies that an amount of damage may be split among some number of units, each unit chosen is targeted. Because Alpha Strike instructs you to "split" damage among enemy units, every enemy unit you select to receive that damage is a target of the spell.

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