If someone reacts to my Kha'zix, and plays a unit, does it get +2 might still?

Yes, your Kha'Zix, Mutating Horror will still receive the +2 Might bonus and gain 2 XP.

Reasoning

According to RiftJudge FAQ #9849, #10176, #10198, and #9937, the condition for Kha'Zix's ability is checked at the exact moment he enters the battlefield and gains the combat designation ("Attacker" or "Defender").

1. Trigger Timing: When you play Kha'Zix, he enters the battlefield and gains his combat designation. At that specific moment, the game evaluates the board state: "Is an enemy unit alone here?"
2. Condition Met: If the opponent has only one unit at that battlefield at that moment, the condition is satisfied.
3. Chain Placement: Once the condition is met, Kha'Zix's triggered ability is placed on the chain.
4. Reaction: If your opponent responds by playing a unit via [Ambush] (or any other [Reaction]), that unit is added to the chain above your trigger.
5. Resolution (LIFO): Because the trigger was successfully added to the chain while the "alone" condition was true, the ability remains valid. Changing the board state (by adding another unit) after the trigger has already been placed on the chain does not retroactively negate the condition or prevent the ability from resolving.

The game does not re-evaluate the "alone" condition when the ability resolves; it only checks if the requirement was met at the time of the trigger.