Let us assume all the above is true. Given that there is no objective measure for "design" and "purpose", as established previously and demonstrable mathematically, of what use is your definition in demonstrating intelligent design?
Remember, just because P implies Q does not make the assertion that Q implies P a valid construction. Elementary first-order logic.
There is no objective measure for "natural" and "supernatural" either. Be that as it may, the common definitions of "design" and "purpose" make the connection with intelligence to be a reasonable one. Not an irrefutable one, to be sure, but it is not a wild stretch by any means.