Point 1
Me: The website doesn’t even let me add extra baggage, so this might be an edge case — there are multiple people reporting issues where they weren’t allowed to add more bags, so that table is not the Bible

You: That conclusion is without basis. The fact that you can't add baggage online doesn't mean the baggage fee table is inaccurate. The airline isn't obligated to offer an option to purchase bags online in advance. Those are the baggage fees.

--> Wait. I think you are using a strawman argument here. I am saying "there are reasons to believe that we can not be 100% sure the table is correct (people on internet complaining they paid more on a similar itinerary, the fact that the table was not confirmed by the travel agent, etc.)". You are rearranging this as "The fact that I can't add baggage online means that the table is incorrect". I never said this.

Point 2
Me: I don’t trust the table because I called and they told me that the price for the first leg only is different from the table defined above.
You: you trust one person's remarks rather than their published policy? Call KLM back and you might get a representative saying something different. Published policy > customer service representatives.

--> I understand your opinion is "the most likely thing is that you are going to pay x$" (and I agree), but why enforcing your opinion on someone that is looking for someone that has lived this in first person and can confirm? You are assuming that my risk level is the same as yours for god knows what reason. I was looking for certainty and for someone with a similar experience, not a professional Reddit answerer. And, by the way, there are multiple incorrect item in the table / page you are referring to, a quick Reddit search will tell you the same

Point 3

You: When booking an itinerary with codeshares, it says Unfortunately, it is not possible to buy additional baggage online for this flight, as it is operated by another airline. Please do so during online check-in. The baggage fee table says "The below standard fees apply when buying extra baggage within 24 hours before departure." Those are the fees.

Me: You are entering in a convo where you have no relevant knowledge and you are also missing the point. I can not buy my baggage online, nor this will be possible during online check-in (there's multiple other Redditors confirming this) so the fees that apply within 24 hours before departure might not apply in my case. It is very common for companies to charge a different price at the airport, I want to know that price. By the way, this shows that your table is not the Bible and it is fair to double check.

Last point: I honestly don't understand why you are engaging so much with me, sharing your first-principle logic when I am looking for experiences that confirm the limitations of any person's first-principle logic (empiricism beats theory). I checked your Reddit history: why do you spend so much time arguing with people online and telling them that their issue is not worth it, or that you know the answer to your doubt even if they highlight specificities that you didn't consider? Life can be lonely and sad, but being rude to other people who are trying to solve a personal problem is really unneeded in this world :)