Disclaimer
As was pointed out by others, it seems not to be a grammatically correct comparison or it is even rarely used in English. Its origin is therefore maybe a translation into English?
Other sources mention that there is a difference of British English ("Much as...") and American English ("As much as...") applicable here:
Apparently the example can only be interpreted as being like a comparison. In German for example, you find such sentences more commonly.
There is a learning video on YouTube which exactly explains the interpretation as I do understand it here.
Take the following as help to understand the meaning of the sentence, but not necessarily as confirmation to build such figures of speech, unless you do it deliberately.
Interpretation
Implicitly there are two "measures" being compared (or better: put into a relation):
- The amount of how much "I like you".
- The amount of how much of "I like you" is needed to "agree with you".
"[As much as] I like her [, although / but], I can't agree with her on this."
It is expressed, that the amount of "I like you" is not sufficient to also "agree" on the topic in question.
Your assumption to substitute with a simple "although" or "but" is correct. See the square brackets in the quote above.
Here are more rewrites on this sentence:
- Although I like you, I can't agree with you.
- I like you, although/but I can't agree with you.
- As much as I like you, I don't like you enough to agree with you.
- I really do like you, you know, but I am sorry, I cannot agree with you.
Discrimination to "real comparisons"
I think the best discrimination is that in a real comparison you have two positive expressions on both sides, which also refer to the same "thing" being compared, while in the example given, there is a positive and a negative/negated expression (because you do not agree) and also a "liking" is put in relation to an "agreeing".
Examples:
I like her[+] as much as she likes me[+].
- Expressing a "normal" comparison and equality of the levels.
- "As much as" could also be "more than" etc.
- If in reality both dislike/hate each other, this will be a case of an ironic or sarcastic statement.
I like her[+] as much as she doesn't like me[-].
- Expressing an equality (or imbalance rather) of opposite levels, "like" and "dis-like".
- There is a "flip" (negation) in the comparison.
- There is some "tragic" or "regretful" connotation to the comparison.
- This is an analogue case for the initial example with "agreeing" (I
like her, but I am not agreeing). In this form it is a figure of speech, mostly used to express something like "pitty", because the levels are not zero but still do not match (they are still not high enough), although it would be nice "if" they matched.
For completeness:
I don't like her[-] as much as she does like me[+].
- See 2. There is an (strong and twisted) aversion expressed in the comparison.
I don't like her[-] as much as she doesn't like me[-].
- Both dislike each other the same.
- Because both sides of the comparison are negated, this case can also be treated as a "real" comparison of equality, like in 1.
- There is not much pitty expressed here, but it could be intended, because the negative form might have been used to emphasize something.