Yes that would be considered an indented natural and yes it would affect the girdle thickness as natural''s are generally not considered part of the girdle unless they are completely confined to the girdle.
So in that instance the girdle may be described as being "very thin" or "extremely thin". Also, in this particular instance the natural serves a similar purpose as the girdle in the sense that a prong can be set over it and in most circumstances not impact the durability of the diamond.
No prob Pyramid. I should have been more clear in that.
What I meant to say regarding naturals and the girdle is that in most circumstances when you see girdles listed as being "very thin" or "extremely thin", it''s generally a problem and does affect potential durability of the diamond in question. However when you have girdles described as "very thin" or extremely thin" and a natural is taking up the portion of the girdle where the girdle would typically be (like in your pic), the natural serves as a buffer like the girdle does and it''s not as bad as it appears when it comes to the durability issues. At least this has been my experience.