Sounds like you need better accountants.
Ahah.
Of course you can declare whatever you wish as a tax exercise. Doesn't mean it's legitimate and would held under close scrutiny by the ATO.
Like if you declare a fee as a business expense, then so should the reward as you get monetary benefit. And then of course, FBT is applicable. There's no free lunch with the ATO.
On my Amex platinum, the 5X reward is for trips booked via amex, and for other expenses, if it's the same promotion I received, it is capped. I seriously doubt you'd get that many points.
Anyhow, good for you.