I’ve had a very quick response to my support ticket, and it basically confirms what @Artisan-West said.
The response was:
"For images like this, you will need to use DeNoise AI first to remove the noise.The noise reduction in Sharpen AI s not the same as DeNoise AI, and vice versa (Sharpen slider in DeNoise AI), and should be considered minor editors. These minor editors don’t use the AI processing models that the AI programs use. We include these options for users who need minor tweaking that doesn’t necessarily require the robust noise reduction of DeNoise AI, and vice versa, but where minor adjustments in another program may prove useful without disrupting the user’s workflow with two separate programs.
I guess I’m not the only one to have expected the noise reduction techniques to be similar in the two AI products, so it is good to have that misconception cleared up.