Nowhere in the rules does it say that if you can take reactions you are no longer surprised. If you want to put that much stock in Mearls' interpretation of the rules, that's fine, but his interpretation is not part of the RAW. Mearls' also interpreted Champion Fighter's Improved Critical as automatically hitting on a roll of 19 even if it would not have been enough to hit otherwise. Not so sure I'm willing to buy into that interpretation either.
As for your assertion that this is a "simple logical understanding of the rules in the book," perhaps you'd care to explain the logic of a target negating surprise when they are unable to act or move on their turn and once their turn ends, can now react to an attack by an assassin the target doesn't know exists.
It doesn't state directly a good many things in the rules. What it does say is.
If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a reaction until that turn ends.
That is it, that is all surprise is/does. So by reading the rules for surprise you can see it limits what you can do on your first turn of combat, and it tells you when you can take reactions. When your turn is over obviously so is surprise.
Mearls statements are not official rules corrections or judgements you are correct, Crawfords are alas we don't have Crawford's response to this question. As a side note, we do have Crawford on champions auto hitting when they score a critical on a 19, and yes they do.
They can react to a target they don't know exists yet because the rules say they can. Let's use a monk as our victim as it fits the example and is your namesake.
A monk is strolling down a road, when suddenly he is ambushed by three ninjas (assassins). The monk is surprised, and initiative is rolled.
The leader assassin goes on 23, a second assassin goes on 18, the monk on 17, and the last assassin goes on 13.
The leader assassin shoots an arrow at the monk, he is surprised and hasn't taken his turn yet. The monk can not use reactions yet. The assassin gets to take full advantage of his class abilities.
The second assassin shoots another arrow at the monk, still surprised because he hasn't had his turn yet the monk is in for a second critical sneak attack if this arrow hits.
The monk goes and takes his turn, he can not move or take any actions on his turn. Once his turn is over he can now make reactions.
The final assassin goes, at this point the monk is no longer surprised, the assassin still benefiting from being hidden though makes an attack with advantage and hits getting in his sneak attack damage. The monk uses deflect missiles class ability as a reaction and reduces the damage.
If the monk would have rolled a 25 on initiative and gone first, they would have been able to use their reaction to deflect the very first arrow that was shot at them because that is what the ability says it can do.
If you can catch an arrow, spend a ki point to throw it back at a ninja, I don't think anyone considers that person surprised anymore.