An API (Application Programming Interface) is a way that let’s one computer program/app/system talk to another. They have what are known as end points. They are options to do different things.
One end point for Todoist uses a single entry for a quick entry. Another one gives you lots of specific ways to forego the natural language and specify each bit of data.
It sounds like the quick entry end point may not yet have been updated with the latest natural language processing. So while you can set it via the app in quick entry and via the detailed API entry point, this could explain why the quick entry API end point does not support the duration… yet.