Let's examine x://
absolute-URI = scheme ":" hier-part [ "?" query ]
hier-part = "//" authority path-abempty
So this is okay if authority and path-abempty can both be empty strings.
authority = [ userinfo "@" ] host [ ":" port ]
host = IP-literal / IPv4address / reg-name
reg-name = *( unreserved / pct-encoded / sub-delims )
path-abempty = *( "/" segment )
Yep.
And the same applies for x:///y, except that path-abempty matches /y
instead of nothing.
This means these are in fact valid urls per RFC3986, counter to your claim.