Does Kha'Zix still get +2 if it initially attacks a lone defender but the opponent reacts with a unit to the trigger?

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

According to the established game rules and several RiftJudge FAQs (including #10198, #10465, and #10735), the condition for Kha'Zix’s triggered ability ("if an enemy unit is alone here") is evaluated at the exact moment he enters the battlefield and gains the "Attacker" or "Defender" combat designation.

Chain Sequence and Resolution

1. Trigger Condition Check: When Kha’Zix moves into the battlefield, he gains the "Attacker" designation. At that exact moment, the game evaluates the battlefield state: Is an enemy unit alone here?
2. Condition Met: Since your opponent had only one unit present at that moment, the condition is satisfied. Kha'Zix’s triggered ability is placed on the chain.
3. Reaction: When your opponent plays a unit in response, that unit is added to the chain above Kha'Zix's trigger.
4. Resolution (LIFO - Last-In, First-Out):
   • The opponent's unit enters the battlefield.
   • Kha'Zix's triggered ability resolves. Because the condition was met at the time the ability was triggered and placed on the chain, the subsequent change to the battlefield state (adding the second unit) does not retroactively negate the trigger.

As confirmed by RiftJudge rulings, 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.