Has the 1.17 Minecraft update been released yet?

Eats

No, not until summer. Only a few development versions

Then why could Wadzee be using it?

As I said. You can play it, but not the full version

It's called a snapshot

No, it will be published between June / July

But there are already snapshots that are like an incomplete version

The latest full version is 1.16.5

As for your other question: "Then why was Wadzee able to use it?"

Activate "Development versions" in the launcher under installations

and then you can choose the 21w13a…

This is the current 1.17 development (it may contain bugs and so on, as it is a development version as I said)