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After reading the optional rule of Hitting Cover, it strikes me that the rule is flawed.

The Hitting Cover variant rule states (DMG p. 272):

When a ranged attack misses a target that has cover, you can use this optional rule to determine whether the cover was struck by the attack.

First, determine whether the attack roll would have hit the protected target without the cover. If the attack roll falls within a range low enough to miss the target but high enough to strike the target if there had been no cover, the object used for cover is struck. If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.

Case

So imagine that I want to hit a target, behind another creature.
For simplicity, we will refer the target as "target", and the creature who is providing cover to the target as "creature".

The target has an AC of 16, and receives a bonus of +2 due to half cover from the creature.

The creature has an AC of 18.

Example

I roll to attack the target, and roll a 17.

I would hit the target without cover, but I do not hit the target with cover: so I hit the creature that is used for cover. (16 < 17 < 18)

But since my attack roll is lower that the creature's AC (17 < 18), I miss him.

Conclusion

Is it not possible at all to hit the creature, as long as the creature providing cover has an AC that is 2 higher than the target's AC?

Your conclusion is correct.

If the Cover's AC is too great, then it won't be possible to hit the Cover incidentally while trying to attack the covered creature. Either you'll roll high enough to hit the Covered Creature, or you'll roll just barely low enough that it'll miss both. In that situation, if you wanted the cover to take damage, you should be targeting it directly.

The rule is mostly designed to handle objects like Walls, where their AC is usually pretty low and intended to crumble + break when used as cover. Obviously, it applies to creatures as well, which means a lot of high AC creatures straight-up cannot be hit if they're not being directly targeted, but that generally makes sense: if a covered creature has an AC of 12+2==14, and gets missed by a 13, you wouldn't expect that to target the Covering Creature with an AC of 20, since that would lead to that creature being inexplicably easier to target just because they have an object behind them. That doesn't make much sense.

Missing AC just means you dealt no damage it doesn't mean you didn't hit

Failing to hit, or 'missing' a target's AC, is just an abstraction to indicate that you did not deal damage to the target. How this occurs varies significantly based on each character, but take the following example for a Barbarian whose AC is 10 + Dex + Con:

Bruto the Barbarian's nips won't quit and has an AC of 18 because his Dex and Con modifiers are both +4. If you shoot an arrow at Bruto and beat an AC of 10, but not 14 Bruto may've simply juked out of the way. If you beat an AC of 14, but not 18, you shot Bruto, but his scarred and calloused body provides a thick hide which your arrow could not effectively penetrate. If you beat an AC of 18, good job! You made Bruto bleed his own blood and he might be kind enough to bring the arrow back to you.

Other characters in heavy armor get to enjoy the benefits of that armor as attacks clang off them ineffectively. While characters in light armor and dex based characters may evade attacks on them or parry them aside.

Regardless, the attack has failed to do damage because you failed to beat the target's AC.

To clarify a bit further, there's a 'hit' and a 'Hit'; the former means you hit something somehow and is mostly just narrative, while the latter means you hit something in a manner that deals damage per the rules. In some situations those terms are synonymous, but not necessarily. Furthermore, there's a 'miss' and a 'Miss', which are usually synonymous but the latter is a rule term indicating a failure to hit a target's AC and might have specific rule implications.

So in the cited portion of the rules you quoted, "...the covering creature is hit." But the covering creature wasn't Hit, so no damage is not necessarily dealt.

The Hitting Cover variant rule may have been designed with non-living cover in mind (i.e. walls or trees) but your scenario doesn't necessarily 'break' it.

A creature's AC is a pretty abstract concept used to determine whether your attack causes damage, but if you swing your sword at a target and miss you can easily interpret that miss as either swinging wide or your sword glancing off the target's armour.

D&D is pretty free with how you interpret the abstract combat rules.

So in your scenario, your weapon misses the original target and "hits" the creature being used for cover. But since you still don't overcome that creature's AC, it harmlessly deflects off their armour (or scales).