Flashback scene in It (2017)

What happens in this scene?

The same article you linked answers this question. We don't know what was in the scene but there are some possible scenes that were in the screenplay.

Early drafts of the “It” screenplay, originally written by director Cary Fukunaga, who departed the project in early 2015, and Chase Palmer, have been thoroughly dissected by the online film community. At least one draft contained a scene featuring Pennywise playing a saloon piano in the 1800s to spur on violence, as well as a colonial-set sequence where It devours a child. Whatever the scene was that Muschietti and company shot, Skarsgard — who wouldn’t go into too many details — said it may yet appear in the sequel.

Is it in the book?

Given that we don't know specifically what the scene was, we again don't know for sure. But if it is one of the ones listed above from the screenplay drafts then the answer is: no.

As far as I'm concerned, your guess is as good as mine. I'm pretty certain that it would be a director's addition because there's no canon in the book which dates back to the 17th century. The earliest I can find are events from the 18th century back when the first colonial settlement at Derry was formally established, as seen in this quote:

"...the original party of white settlers numbered about three hundred. They were English... The land granted them covered what is today Derry, most of Newport, and little slices of the surrounding towns. And in the year 1741 everyone in Derry Township just disappeared." It, Derry: The First Interlude

Upon looking at the source you linked, I noticed that the scene concerned:

"There was a scene we shot that was a flashback from the 1600s, before Pennywise [was Pennywise],” Skarsgard said. “The scene turned out really, really disturbing. And I’m not the clown. I look more like myself. It’s very disturbing, and sort of a backstory for what It is, or where Pennywise came from. That might be something worth exploring in the second one. The idea is the ‘It’ entity was dormant for thousands and thousands of years. The [flashback] scene hints on that."

I think that this suggests that the scene might have concerned (similar to the novel) a group of settlers coming across the virgin lands of modern-day Derry; potentially, being some of the first people that had accidentally stumbled across It in a long while. However, how this interaction might have been depicted is open to speculation.