Is this testing the spending limit, just an estimate? or it is for sure it is gonna go through?
Since you have "no pre-set spending limit", this feature allows you to make a test charge to see if it gets approved. I believe this number also changes based on your spending behavior and longetivity with AmEx
I just checked my spending limit of my Gold Card and it said I would be ok for a $40,000 charge (didn't check higher). But my experience with AMEX is if I charged that kind of amount they would be 'asking questions', at least to verify identity and possibly to verify funds and banking information. And if I was trying to charge that kind of money at a pawn shop, casino or other potentially eye-raising place further hurdles would pop up. Look at the tool on AMEX as a guide, but I wouldn't put an iron clad guarantee on it.