This twist feels like it was pulled out of the ass by tye author writing themselves into a corner. The art this chapter didnt look like that way back then, it just looked like a slime thing.
As Barney as the plot has been so far, the author has been dropping many hints since the earliest chapters that this world is a constructed one and that it was by someone with knowledge of Japan, likely Izanagi.
Additionally every thing has a reason, no matter how crazy it is. I found it strange that everything pretty much has had an explanation except for the slime attack, which was really odd. It wasn't an ordinary slime, it had a face, and he was seemingly being absorbed before teleporting away.
There are a ton of convenient circumstances in the manga. But that's different from an incident that has no connection to anything whatsoever.
The the author has drawn the same characters more different than this scene and the original from chapter 3 in other places, sometimes in adjacent panels. It's a stylistic choice to make the slime less human like before but more obviously him this time around. I wouldn't use that as the sole reason to accuse the author of making this up on the spot.