Found it.
By entering docker exec -u www-data -it devkinsta_fpm bash into the OS Shell one is entered into the Docker container, and the www bit of cd /www/kinsta/public/siteName has to do with server/Docker things and does not exist in the real OS filesystem.
It’s probably the Container thingy that threw me off here; as a user of MAMP Pro, the newer «lightning» LocalWP builds and currently Laravel Valet this is an additional complication.