If I move Kha'Zix, Mutating Horror to a battlefield with just put Rookie, and at reaction to my Kha'Zix trigger, they ambush Rengar to the battlefield, does my Kha'Zix trigger still go through?

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

Reasoning

According to the rules for triggered abilities and established RiftJudge FAQs (e.g., #9849, #10198, #10748), the "alone" condition is checked at the exact moment the unit gains the "Attacker" or "Defender" combat designation.

1. Trigger Condition: When you move Kha'Zix, Mutating Horror to the battlefield, he gains the "Attacker" designation. At that exact moment, the game checks the battlefield: Is an enemy unit alone here? Since your opponent only controls the Pit Rookie, the condition is satisfied.
2. Trigger Placement: Kha'Zix's triggered ability is placed on the chain.
3. Reaction: Your opponent plays Rengar, Pouncing using [Ambush] as a [Reaction]. This adds Rengar to the chain, resolving before Kha'Zix's trigger.
4. Resolution (LIFO): Rengar enters the battlefield. Then, Kha'Zix's triggered ability resolves. Because the condition was already met at the moment Kha'Zix gained the "Attacker" designation, the game does not re-evaluate the battlefield state when the ability resolves. The change to the board state caused by Rengar does not retroactively negate the trigger.