That's nonsense. You probably mean TTL logic family. That's not the same as digital logic. Also 'X' in HDL doesn't mean open circuit which is represented by 'Z'.In digital logic when input of a gate is X it is like high.
<Unknown> XOR "1" = <Opposite of Unknown>
<Unknown> XOR "0" = <Unknown>
<Unknown> XOR <Unknown(which can be either 1 or 0)> = <Unknown or Opposite of Unknown... simply Unknown>
I don't see an indication for this assumption.Reading the original it appeared the same input applied to both inputs.