This end up

There's only two exceptions where he acts like Jack is no big threat:

one time he got mad he couldn't get 8-year-olds to like his fairy tales and angrily declared "an almighty, all-powerful wizard" destroyed "a pathetic, little samurai" before storming off

one time he told his scientists "he really doesn't care about a pathetic samurai who wanders around like a mad fool" because he's trying to hide that he's been driven into a deep depression over failing to kill Jack for fifty years.

But the rest of the time?

The moment Jack arrives in the future, Aku freely lets everybody know that he considers Jack an actual threat, one so dangerous he offers bounty hunters more money than exists in the universe if THEY kill Jack FOR him.

Aku is infamously immortal and indestructible—and yet he makes no apparent efforts to prevent his citizens (who hate him) from learning that Jack's sword can kill him. We don't know how fast that info spreads, but at least by the 50-year time skip Scaramouche confidently identifies the sword as "the one thing in the entire universe that can destroy Aku."

More than once he claims nothing can harm him and then is immediately harmed by the sword; each time, there's no denial—no "WHAT?! THAT CANNOT BE!!"—and no fumbling to cover up his error. He just starts inspecting the thing that just harmed him.

Demongo says, "You are a powerful foe. Perhaps too powerful for my master… but not for me." It's possible he only claims to be stronger than Aku when he's safely away from him; but, given how long he seems to have worked for Aku (and given that Demongo must know Aku could easily be watching the battle), it seems likely that they both understand that, by summoning Demongo, Aku's saying "I can't take out this guy, maybe you can."

There's never an "I would have defeated you if it wasn't for—"

Never an "I think you've had enough, I'll go easy on you by leaving now."

Never an "I won't fight you because you're beneath me and not worth it."

Instead, he says, "Soon we will fight. But not until I find a way to defeat you and your sword."

He says, "You might have beaten me now, but I will destroy you in the future."

He says, "No, I will not fight you today. Ugh, I don't feel good..."

Over and over, he's not ashamed to admit—to Jack, to his underlings, to the whole world—that Jack is a threat Aku can't currently beat himself.

And that's SO RARE for a villain like Aku! Big powerful dictator who's used to being worshiped, who makes up big fancy titles for himself, who brutally punishes anyone who who doesn't grovel at his roots? Usually villains like that are WILDLY insecure. Most similar villains would never admit vulnerability. Would be INCENSED if someone dare pointed out they typically lose to the hero.

Instead Aku consistently points it out himself!

It's obvious Aku's got a huge ego. Huge enough to think he deserves to be master of the whole planet. And a world-dominatingly huge ego usually means an inflated ego: a villain fooling themself into thinking they're stronger, cleverer, more competent than they are, and getting outraged when that's challenged. Such villains have to believe they're flawless or they crumble.