Is there some evidence that American Express ignores history on other cards?
On strictly their charge cards, Amex seems to care more about internal information (on time payments, how account is managed...).
OTOH for credit cards or any of Amex's financing options such as 'pay over time"... then yes, Amex does look at overall credit score and history. This will likely happen when annual or some other event prompts a review and Amex does a soft of hard pull of credit report.
If they see signs of fiscal distress then Amex behaves like any other issuer or credit with everything on the table including limits reduced, etc....
Friend used (against my advice) Amex's "pay over time feature" about a year ago. Not understanding how the thing worked, and that you really cannot get at the POT balance without either not using card or making hefty payments well over current balance due, he's been nibbling at it ever since.
Down turn in his business last summer put a bit of limit on financial resources. He was paying things on time, and more than minimum for all cards, but something spooked Amex. He got a letter several months ago (before covid-19 hit) that Amex had chopped his POT limit way down. Reasons given were he was taking too long in paying the thing off, and that there was "information regarding other credit accounts) when Amex pulled his credit report. They went on to give standard blah, blah, blah about who they pulled credit report from, his rights, how to correct incorrect information....
All this went down about a month or so after his annual fee hit, and from credit report Amex had pulled a credit report at that time as well.
Since Amex charge cards don't have preset spending limits how much any account holder can charge per transaction (or per billing cycle) is something they themselves only know. Amex does state when making decisions about extending credit they are based in part on "internal information".