If you're expecting some leaked internal document that says "priorities: 1. benchmarks, 2. security", no, that probably doesn't exist. But their priorities are revealed by their actions.

GrapheneOS typically ships security updates within a day. But most of Samsung's phones get quarterly security updates[1]. There will never be a primary source with a Samsung insider saying "we're fine with our customers being vulnerable to widely exploited security bugs for the next 89 days, in order to save our massive corporation a few thousand dollars a month." But their priorities are revealed by their actions.

If the statement "OEMs don't care about security" is trivially proven false, as you said, then you should be able to prove it trivially by naming even just a single OEM whose update cadence matches GrapheneOS's.

[1]: https://security.samsungmobile.com/workScope.smsb

Also look at their advertising. Endless "X% faster" mentions that often get significant visual space, but when was the last time you saw "security updates 7 days earlier" or "tagged memory for better safety"? Even a single mention of "security" on a phone's manufacturer page is uncommon, and that's usually just mentioning a normal Android feature, or something like Knox or Secure Folder and not "we made choices that make a common class of attacks much more difficult or impossible".

The numbers aren't zero, certainly. Nor does it imply directly proportional levels of effort. But they are incredibly obviously given wholly different levels of attention on nearly all phones, by nearly all phone manufacturers.