I agree with those who feel this is not just a resized JPEG straight out of the camera. Not that I wish to doubt you, but the information contained in the EXIF data suggests this image was processed in an external editor.
For example it appears that the ProPhotoRGB color space was applied somewhere along the way. That alone would be problematic for an 8-bit highly compressed file. That without question would cause posterization with fine gradient areas. Also ProPhotoRGB is not an in camera choice.
But there are other troubling things that would cause issues. You seem to have a -3 2/3rds EC dialed in. It also suggests you used flash, but I don't know if it was the built-in or external flash. But never the less, with the exposure settings you have unless the ambient light was sufficient, I don't believe there would be enough flash power outputed to properly expose the full scene. That suggests that you might have needed to boost the levels in PP which would seem to increase the visible noise.
I'll take you for your word that you did no external editing, but the information in the EXIF data seems to contradict you.