The screenshot above indicates that the stealth ship is visible to the naked eye to someone on HMS Devonshire.
The screenshot indicates that it's visible to the movie audience. The ship is only visible to us for storytelling purposes
In this case we, as the audience, have a better view that any watchkeeper. In the dead of night seeing a black or dark grey object at any distance without a fairly large light source is next to impossible.
Even if not, couldn't HMS Devonshire have detected the stealth ship some other way?
I think that's the point, it's a stealth ship designed to be essentially invisible.
Could the Devonshire have detected it some other way, possibly but suspension of disbelief is inherent to movie story telling. You just have to go with it.