Why don't we call this stream Underune since we talk about Undertale and Deltarune?

Deltarune can't take place in the Undertale timeline because Deltarune is just an experiment created by someone in the Undertale timeline.

Yes

Also Toby Fox himself said they don't take place in the same universe

One of the most widely accepted theories is this.
Gaster made Deltarune as a world without choice, because if the PLAYER is free to choose they will eventually turn to Genocide. This worked for a while, but at the end of Deltarune Chapter 1, the evil spirit of Determination forced its way into the world and corrupted Kris.

Since Determination leads to choice, and choice leads to Genocide...
We can all assume what happens next.

But hey, that's just a theory. A Gay-Game Theory!

yes