You're absolutely right and I have no idea how to best solve this communication problem. The rest of the world is simply trained to scope box. They probably don't even know what their technique is called.
I've found time boxing works much better the deeper in the bowels of an organization you work in.
If you have a receptive customer, try and start with a regular release cycle as part of the overall project management. "During development, we'll do regular releases every x-number of weeks. We'll meet every 2 release cycles to see where we are with respect to overall scope vs. the last release(s), spot bottlenecks or readjust scope and requirements as needed". This gives them lots of input into the process without getting too involved in the day-to-day and let's you timebox. They get regular insight and feedback as the product progresses, and you can emphasize or deemphasize requirements as things go on.
I've found time boxing works much better the deeper in the bowels of an organization you work in.
If you have a receptive customer, try and start with a regular release cycle as part of the overall project management. "During development, we'll do regular releases every x-number of weeks. We'll meet every 2 release cycles to see where we are with respect to overall scope vs. the last release(s), spot bottlenecks or readjust scope and requirements as needed". This gives them lots of input into the process without getting too involved in the day-to-day and let's you timebox. They get regular insight and feedback as the product progresses, and you can emphasize or deemphasize requirements as things go on.