I believe the cancellation of the visa was stated to be on the grounds that his presence in the country was contrary to Australia's interests in that it would tend to encourage people not to get vaccinated.
I think there were two reasons given : one on public health grounds, and the other as you suggest.
Both of which are clearly nonsense. It is the behaviour of the Australian authorities - first in granting him a visa, then cancelling it despite him having followed all the rules they set, and now this seemingly-arbitrary exercise of executive power, that have drawn his case to such massive attention. It looks petty and ridiculous, and making him a 'martyr' like this doesn't seem at all likely to help their case.
I find it very uncomfortable that a minister should have the power to make arbitrary decisions like this, independent of what the rest of the law actually says. There's a lot of similar things been written into our laws recently, and they make me uncomfortable too.