I have signing authority for my department [30000+ employees in corp] to approve Amex applications and etc and I get contacted if an employee applying for a corporate amex gets denied due to their personal credit rating not being "good" enough for amex to process without me/we signing a specific guarantee for that specific employee.
Also, I have had employees terminate without submitting expense reports for charges on their corporate cards and I know that Amex makes their lives a living h*ll without so much as Amex calling me/us to make amends [I know it happens because the former employees have been the ones to call for help in resolving]
With that said, in my experience, in my company, Amex corporate cards still have a personal responsibility to them and probably can count towards total credit extended from an Amex point of view.