The Tuya plugin sadly is using the cloud. Fwiw, you can see whether a plugin works fully local on our integrations page: https://nymea.io/documentation/overview/integrations
Watch out for the cloud icon on the tiles.
At this point, tuya does not provide any API for their devices. Neither local nor cloud based.
There is a cloud API  at courtesy of the homeassistant folks implemented as a tuya “skill”. I couldn’t find any official documentation for that but the client code examples allowed to get it working with that API in a user friendly way.
While there is an approach to reverse engineer the local API , at this point it would require the end user to dissect the tuya app on android to extract encryption keys. As this is not something most users can do, I decided to go for the cloud API at first.
A plugin for the local API also shouldn’t be hard, probably most code of this repo could be taken and converted into a nymea python plugin. Maybe someone is interested in giving that a go?
You could also try and see if you can reflash those tuya devices with tasmota to completely remove the tuya cloud from the picture. An example for that would be here .