No preset spending limit is not the same thing as no spending limit. Everyone has some limit in Amex's system after which they will not let you spend any more on a charge card, and I suspect for someone getting a charge card with very little other credit history, that limit is fairly low, though it may still be higher than the limit such a person would likely get on a credit card since a charge card in theory puts the issuer less at risk.

You can get a sense of your own limit by going to "Check Spending Power" in the Account Services section of the Amex website. I checked whether Amex would let me spend $20K and it informed me that yes, they would, and even specified what my Platinum charge card limit is exactly, which is apparently in the mid-five digits. (There may be a bit of leeway to extend that limit if you call them, but the same is likely true of many credit cards, and will depend very much on your history with the issuer.)

I guess the only thing that makes this not a "preset" spending limit is that it in theory moves around more often than a credit card limit does? I'm not really sure. Plus there's the leeway factor.