Afraid I fall in the camp of finding this AA.com statement in fact quite unclear and I always find these statements frustrating when plotting trips requiring codeshare flights (particularly Fiji)

A sentence that reads as above (...may not accrue...) seems to leave me with several options, all ambiguous. It could mean that I may receive mileage accrual on non-oneworld carrier A, but not on non-oneworld carrier B. Or even more frustrating, I may recieve mileage accrual on non-oneworld carrier A on one occasion, but not on another occasion. The sentence implies that the whole system is luck and happenstance.

Now of course flying today is perhaps a bit of "luck and happenstance," I know! But this area should not be luck or happenstance. Mileage accrual rules are of course coded into the IT/computer systems of the various airlines. They are not "fuzzy" rules, that's not how computers work. For any possible combination of airlines your flying (i.e. AA, Oneworld, non-oneworld, etc.) those systems look at their rules and choose to credit or not credit, but make no mistake, it's not random (now I'll grant that computers make mistakes, I get that; meaning someone not supposed to receive mileage does in fact get mileage. But a computer screw-up does NOT mean a set of objective rules doesn't in fact exist)

However, the various carriers, and in our specific case, AA seem to choose not to simply clearly state what the rules are for each specific carrier; one-world, or a oneworld partner that isn't themselves oneworld will do.

To me, to make the AA.com sentence above unambigious would actually be quite simple and I have no idea why they don't do it. They could mearly reword it to state:

"Flight mileage will not accrue on flights operated by carriers other than oneworld carriers."

Then if someone still books said flight and the computer gods smile upon you and credit with mileage, well your pleasantly suprised as you were not expecting it.

Just my two cents.

Regards.