This thought experiment doesn't have to involve trucks: most small businesses involve owners working in some capacity, even if it's only managerial (but productive management is labor same as any other component that is required to produce the final product).
Thus, they're being a laborer when they perform productive work, and they're being a capitalist when they pocket the wealth that business as a whole (i.e. all employers collectively, rather than just themselves) have generated. And if they don't actually do the latter, there's no economic exploitation involved.
You said it better than I. I don't think I made my point about "context" sufficiently clear above. A person's capital can be earning more capital at the same time as they are employed in productive work, but not in the same context. An owner of a small business may "pay themselves a salary" in accordance the salaries of their employees, but that doesn't make them "not a capitalist" at any point in time insofar as they own the company and employ people to work at that company who do not own it.
Thus, they're being a laborer when they perform productive work, and they're being a capitalist when they pocket the wealth that business as a whole (i.e. all employers collectively, rather than just themselves) have generated. And if they don't actually do the latter, there's no economic exploitation involved.