Glass mixologist trigger can target deathknell unit on your bf that died at same time and why?

Yes — provided the death happens at a battlefield you control (i.e., you're the defender or it's your battlefield, not one you're attacking into).

When two units die at the same time, each Deathknell trigger is added to the chain as a Pending Item before the unit actually moves to the trash, and the game notes its attributes and location (Rules 808.1.d.2–.3, 428.1.a.1.b). Since an item is on the chain, you also can't lose control of the battlefield during the following cleanup — so the graveyard unit is a legal choice and the battlefield is a legal destination. You control both triggers, so you choose their order (Rule 808.2.a): resolve Glasc Mixologist's first, play the just-died unit from your trash (as long as it meets Glasc's cost limit), and it enters at that same battlefield.

The why: Deathknell "remembers" state from before death, and the pending chain item freezes control of the battlefield until the trigger resolves (FAQ #9783, and the Unleashed clarification that control can't be lost while an item is pending). Caveat: if you're the attacker at an opponent's battlefield, you don't control it, so you can't play the unit there (FAQ #10612).