Not necessarily. They might know without having enough evidence to stand up in a prosecution.
Pokey, the people I heard the story from weren't focussed on the motivation for the release nor trying to give it an anti-hunter spin, their interest was in the eradication effort.
Given how efficiently rumours spread down here, I think it's highly unlikely that any individual or group suspected of deliberately introducing a breeding population of foxes into Tasmania would remain an anonymous "they" for very long, even if the authorities, themselves, lacked the evidence to actually prosecute.