I have played it and I enjoyed it! It doesn't go as deep as Undertale, but the characters were nice and memorable. I enjoyed the mechanics.
Yeah, its not really a sequel. It's an entirely different timeline. It was actually in Toby's mind way before Undertale was. If you think about it, Undertale is more of a spin-off of Deltarune than the other way around. Undertale was a project where he wanted to prove that he could design and program a game all by himself, and the game itself was released in 2015... But Deltarune was in the works since 2012. Yeah.
And it's only a "demo" of sorts, too. He said on his twitter that he has it all planned out but might actually not be able to tackle it all alone this time and is looking for a team, but no kickstarter this time. So we might see the continuation in.... he doesn't know when. Apparently he doesn't really want to work on it for longer than 7 years, so we'll see.
I do hope that when the continuation does come out, I hope that Alphys and Undyne get together again, as well as Mettaton gains a body he is happy with again in that timeline. Those were the saddest part for me imo.