Well, it is the truth. It won't be done before it is done. It is understandable that there is a business that needs to function, but the issue here is the question of asking for an estimate like you've already solved the problem, instead of actually sitting down with the engineer to discuss the business problems that need to be solved. That's what engineers are there for: To solve business problems. Estimates are irrelevant as the solution will be designed with the business constraints in mind.
> it's quite doable to break a large project down into a series of sprints
This too comes across like the problem is already solved. You don't need to break problems down into sprints. That is a ridiculous way to operate. This kind of thing only shows up where there is some weird effort to separate engineers from their jobs.
In fact, "sprint" comes from Scrum, which was designed to be a transitionary exercise to get engineers more comfortable with Agile, which is all about removal of managers. It is intended to teach engineers to think and act more like managers so that when you get rid of the managers completely that they don't flounder. If you are doing it as more than a temporary thing, you've entirely missed the point.