dev.to → URI
dev.to with protocol (https, ftp) → URL
Is this right ?
dev.to is not a URI, it is just a domain name.
URI's must start with a scheme (e.g. file, http, ftp) as specified in section 1.1.1 of the RFC you linked to.
A scheme may or may not be associated with a protocol which is where your confustion may be coming from. For instance the schemes 'http' and 'ftp' are clearly associated with the HTTP and FTP protocols, whereas the 'file' scheme is not associated with any protocol.