From the wiki:

Use the place=city tag to identify the largest settlement or settlements within a territory, including national, state and provincial capitals, and other major conurbations.

Truro is the provincial capital and largest settlement in Cornwall, having visited it the signs on the ground say it is a city. While it might not meet the criteria on some basis it's still the largest settlement in a less populated area, its hardly crowding the map.

The violation is not from my edit, but from the previous armchair mapper who had bulk edited all city and town nodes in the UK to follow some arbitrary threshold.

A much more valid complaint would be about Wells and St David's, but for the sake of two village sized cities, isn't going to result in a cluttered map and even though they are small they're probably still useful reference point for getting a bearing (especially St David's, is in such a remote point it's the only landmark).

The world is full of historical quirks and to map it other that it is would be to admit that aesthetic is more important than accuracy and given the link between OSM, wikidata and Wikipedia are you also going to convince those projects that their city lists should also be changed? If you achieve that then you'll just have to convince the King and the UK government to change it as well. I found and fix this revision after using OSM as a database to render a map of all UK cities and was surprised to see the list was missing some and had some towns instead.

Ah add the address then, the street complete app was asking if it had a house number for which I answered no.