Unfortunately, this does not work with this tool, because it is more intended for exchanging words or already existing fonts. It does not even allow you to enter a non-existing font in the search mask.
The idea of the above mentioned feature, however, is a completely different one: namely, that a font, which is embedded in the document, but the paths to it are missing or it is not even on the computer, can be relinked or replaced by another, new font and that for the whole document.
It should then work like this:
Affinity Publisher detects a missing font in the document and reports this. Now the user can use the "Manage Fonts" tool to replace the missing font with another one. The tool already offers the mask, but the "replace" button and the action that applies this command to the whole document is missing. Either this is a bug or the tool is not programmed to the end.