Common mistake. Atheism can't mean both of those, without a specifier; hence, has to mean the only sensible option, which is 'disbelief'. Again, there are reasons for this, one of which is that, you are consequently 'adding' inference, to some ones ''I don't know''. Not everyone want's an added inference, /the added inference is actually unavoidable without other probably arbitrary or redundant descriptors/.
If you think that it can mean both with added descriptors, it loses inherent meaning, altogether.
ex.
using your format
/agnostic theism=contradictory
The real problem here is that the definitions don't quite match the usage, /necessarily. That is why you have to use the definitions that are 'constant', regardless of the variables.
the fact is, ''I don't know'', is not ''atheism''.
//atheism posits an oppositional stance, as mentioned earlier.
If atheism is used as only a non-position, then it can't be an oppositional to theism, as well,/ non-positions aren't oppositionals, they don't infer a position/adherence.