I'm not really sure why we're complicating things with the shell argument. It seemed to stem from wanting to prove Bowser had narrowly escaped great harm and so he was tucked in his shell as a defence mechanism
a la the overused NSMB Koopaling tactic but ended up making it questionably how the feat should be treated due to him being more durable in said state. But... I feel there's an issue with looking at the still image scan posted. You can just about see Bowser's hair and arms, they're just being obscured a bit by the flowers in front of him. It's much clearer in the video footage, and so it seems he's just lying on his stomach. His head
kind of looks like it comes out of the shell as he gets up, so I'm not going to try to discredit this idea entirely, but I think it's really just the animation being a bit weird and it's just him going from looking down at to floor to looking up as he gets on his feet. Basically, I feel like bringing this idea up has only just complicated things...
Bowser's shell seems pretty durable, though. For example, in Paper Jam Bros,
the Bowsers will use their shells as a means of forcing you to have to focus on the other, even being unharmed by Bros Attacks, and
even when hitting it numerous times with their hammers Baby Mario, Baby Luigi and Baby Bowser don't dent the shell at all in Partners in time. Plus
it's a defence mechanism in NSMBU. So... from memory, I don't think Mario has actually harmed Bowser in such a state. I see whilst typing this Blaze has brought up the bottom of the shell is vulnerable, but as brought up earlier, in SMG, Bowser tucking up in his shell has spikes on both the top and bottom, so he's more or less shielded his weak point too... But yeah, in pointing that out, the scan implying Bowser tucked into his shell in the final cutscene looks even less like that, so I feel like this could issue probably be avoided.