It's not a matter of *being valid*.
It's a matter of being Truth. And those verses do NOT give people license to make stuff up years or centuries after the fact and pass it off as truth.
That’s some very strained word parsing you have going on. “Valid” versus “true” — really? All that stuff that Jesus did which John didn’t write down in his Gospel: is it somehow not both “valid” and “true” because it’s not written in there? If you think it changes anything, I’ll add one more idea to what I said before: Something doesn’t have to be in the Bible to be Truth.