No, they aren't. I did not say "A person's tested genetic code never changes", I said exactly what I meant, which is that genetic codes can be changed. Retroviruses do it. Further, on a grand scale, chimeras [1] exist, with human chimeras often entirely unaware of it (while my priors would suggest I am not one, I can't even come close to proving that to you with the rather meagre medical tests I've had done on me), and on a smaller scale, small mutations in cells can easily propagate out into the body over time, even before we ignore the matter of cancer.

Most of the time, this doesn't matter, but if we're talking about taking hashes for security, suddenly it does.

23andMe is not going to provide you with an "authoritative" DNA "fingerprint" because how would they tell that this genetic sequence in this chromosome from this cell is legit but this contradictory sequence in this other cell is different.

Obviously noone has the capability to do this, so they present you instead with "most likely" results. Not the level of proof I'd be looking for in a court of law.