The problem was to get the promise out of the iframe when you shouldn't do this directly in the first place.
This literally is an XY problem: "I need to do A but it's giving me bad results, what do I need to add?" - "Don't use A, it's bad practice. Use B instead and keep using built-in tools instead of hacking something together" In this case use instanceof instead of is-promise because it's a hack around the actual problem of getting objects out of a different context that was explicitly designed to behave this way.
I'm afraid that you don't know what an XY problem is.
JavaScript developers always seem to think they are the smart ones after their 6 weeks of some random bootcamp and then you end up with some crap like NPM where a single line in a package out of hundreds maintained by amateurs can break everybody's development environment.
