It still is a monad. It's just Zig doesn't have language support for monads, so it's less ergonomic.
Just as modular addition over ints in Zig forms a group, even if Zig has no notion of groups. It's just a property of the construct.
Laziness has nothing to do with it.
What that means practically for Zig, I'm unsure.
1. Yes, Zig is doing basically the same thing as Haskell
2. No, it's not a monad in Zig because it's an imperative language.