While Netherite has the same amount of "Armour" as Diamond, Netherite has 3 "Armour Toughness" per piece while Diamond only has 2. This means that you do actually take (slightly) less damage.
Plus, Netherite tools/weapons do more damage than Diamond, too. Defence isn't the only important factor. And, while rarely useful, Netherite items don't melt if you die in lava.
Even though the difference may be small, a Netherite armour/tool is just objectively better than its Diamond counterpart (unless you care about repairing it in an anvil, which the Mending enchantment obsoletes).
But I think that a lot of the hype is less about "Mojang added something that's a little better than Diamond", and more about "Mojang actually added something that's a little better than Diamond after so many years, which proves that they might be willing to do that again in the future as well. What will be the next thing that they do this with?".