What IO is probably (I'm not a mind reader) suggesting is two things:
1. Declaring the costs of the flight and then getting reimbursed might (note: might) be beyond the limit that's set by the tax authorities as a reasonable claim per mile. Any excess above that is taxed. At least, that would be the case in the Netherlands.
2. According to the ANO article you can only claim the direct costs of the flight. Not the annual costs. But in a rental scenario, annual costs (insurance, hangarage) are all part of the rental costs. I think IO claimed earlier somewhere on here that nobody has established whether this part of the rental costs can be claimed or not. If yes, then the law is unfair to owners of private planes and if no, how do you determine what % of the rental costs are annual costs, and what % is variable?
Both of these are most likely solved if the company picks up the bill for the rental, and let you fly the plane. Subject to the other limitations in the ANO article of course.