No that's not at all what I meant. I'll try to rephrase it. Kant thought that we could distinguish analytic judgements from synthetic ones just by mentally examining the concepts involved. My claim is that if that were true, we would expect there to be near-unanimity about which kind a particular judgement is, just as there is unanimity about "all bachelors are unmarried" being analytic (among those who accept the distinction between analytic and synthetic). Mathematical equations have been claimed as both synthetic and analytic by significant groups (ie. not just isolated cranks, like people who claim that time is an illusion), which makes Kant's claim at least questionable.

I'm not sure quite what you mean by 'nihilism'. (It seems to be able to mean anything that the person writing doesn't like.) But in any case the argument I gave above doesn't apply to natural science because natural science doesn't claim intuitive knowledge of its objects. Since natural science studies non-mental objects, scientists' disagreeing is just an indication that there's more to be learned about the object in question. Kant's doctrine on analytic/synthetic judgments, on the other hand, says that we only have to mentally examine our concepts in order distinguish one kind of judgement from the other; ie. it's not a question of collecting more data, as it is in natural science.

If I say that mathematical equations are analytic, and you say they're synthetic (I'm just using this for exposition; I'm not taking one side or the other), and the source of my knowledge is introspection, then how can I explain your disagreement? I would have to a) claim that you're deliberately misrepresenting the results of your introspection; or b) claim that you are somehow less able to introspect than I am; or c) claim that you don't have the same concepts as I do. Obviously a) and b) are pretty weak responses, since you could say the exact same thing to me and we would be at an impass. One of your suggestions ("I suggest mathematical equations are [...] interpreted differently by various groups, and so they form different judgmenents i.e. their usage of the symbols differ") seems to be something like c); the problem with this proposal is that the only reason to believe that our concepts differ is our disagreement. Our concepts of numbers agree when we're just using them (ie. to perform mathematical operations) rather than talking about them, so the only difference between my concepts and yours is that I think mine make analytic judgments and you think yours make synthetic judgements. (Again, I'm just using this as an example; I'm not taking sides.) Describing a judgement as 'analytic' or 'synthetic' would then say nothing about it, since it would have exactly the same use either way. Ascribing the disagreement to our having different concepts just makes the concepts meaningless.

Your second suggestion ("I suggest that they [the groups who claim it is analytical] are not even plain wrong, but unqualified to make a judgment") looks like my b) above. The question here is how is it that you have such a superior analyticity-detecting ability? How can I know that you're the one who has it, and not them? And as for the "definitions of the symbols", doesn't defining numbers as "1+1+1+..." make mathematical equations analytic? If 2 is defined as 1+1, and 4 is defined as 1+1+1+1, then 2+2=4 would just be a tautology, 1+1+1+1=1+1+1+1. (Philosophy of math isn't really a big interest for me, so though I'm sure someone has come up with an objection to this already, I don't know it.)

If, instead of assuming analyticity can be determined by introspection, we look at concepts as normative standards for the use of words, then this disagreement isn't problematic. It could either be that we don't know enough about the norms in question (just as natural scientists disagree about objects they don't know enough about yet); or it could be that the norm doesn't cover this particular case, and then it would be philosophy's job to establish a norm, if needed (just as the courts in a common-law system don't just determine whether this particular case falls under the concept of, eg. 'theft', but also change the concept of 'theft' by deciding what cases fall under it).