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.