That clause is at the very end of the 14th amendment, section 5, and they are cherry picking, this is extra power for congress if they want to make additional laws to help enforce it. However the 14th amendment has four sections prior to this (not just the section on insurrectionists) and these are often enforced even without specific additional laws, such as equal protection, through the courts. Sure congress can make specific laws helping to enforce equal protection, but they don’t have to, you could always sue in the court if your constitutional rights were violated.

The clause about congress may make laws is also included in other portions of the constitution. The reason it’s added is without it, it might be unclear if federal congress would be able to make any additional law pertaining to this beyond what is already written in the amendment, due to the tenth amendment which states that any powers not specifically delegated to the federal government fall to the states. This reasoning is present in the still recorded debates to get the amendment passed in the first place, when someone brought up is it necessary to even add that clause at the end. So if say congress wants to make more laws to help protect voting rights (also protected in the 14th amendment), they now have the power to do so and can cite that clause for why they are able to make this law under the constitution, but what is already written in the amendment does not vanish without more laws, it’s nonsensical.

The liberal justices argue (correctly imo) that limiting just this portion of the amendment to be non active at all unless congress makes a specific law makes no sense, is not how it was used historically, and goes too far. The conservative justices are willfully misreading this to try and shut off any potential route to using the 14th amendment as intended here, such as through the federal courts.

Your argument (and the conservatives on the supreme court) would be saying because of this clause the entire fourteenth amendment, sections 1 through 4, (and any other part of the constitution that uses this language, including the thirteenth amendment banning slavery) are totally inactive unless congress decides to make a specific law about it. So no ability to sue for violation of equal protection? No protection of voting rights unless congress decides to make a law about it? No ban on slavery if congress doesn’t get around to passing something? What kind of constitutional protection is that, if it can just be stripped away simply by congressional inaction? And how does that make any sense when for this specific clause it goes out of its way to say a 2/3rds majority of congress is needed to override it? The conservatives arugment effectively totally nullifies this and makes it a simple majority to over ride it.

There’s no reason to isolate just this portion of the amendment, and say this part is inactive without a specific law, unless they’re a partisan hack like most of the supreme court, trying to provide cover for individuals who made an attempted coup. Or more lazily just side stepping the responsibilities of the court. Even Barrett thought the other conservative justices were going too far.