FWIW AA hugely prioritizes % of premium fares purchased here. I've made CK every year on relatively low aggregate fare spend (and almost no credit card spend) because I am 100% refundable premium fares. I've had my CK reps at my home base confirm that I'm basically a shoe-in because that is the most relevant metric. I also (from the CK board) know of plenty of AA flyers with nearly 500K+ in CC spend that haven't qualified for CK because of a dearth of premium fares in their booking patterns.
That doesn't imply that you're wrong about CC spend being a factor (and maybe an increasingly important one for airlines as their card programs continue to become more profitable) but wanted to add my anecdotal knowledge here given in the FT spirit!
Correct, the metrics aren't linear. But there are reports of people with 1M+ LPs just from cc spending that got the CK invite. The cutoff isn't published of course and can vary by year.