I suspect the ham with the tower has overlooked a few vital things here. Concrete is a material with specific properties. The vital ones for tower support, or in foundations, or things like lamp posts, electricity poles etc is that these all support tall structures that want to create turning moments when they are in motion - from things like wind, or rotating different top loads. The vertical can be set in concrete, or can have long studs or rebar like extensions that get set in concrete. What works for you is mass and weight and the resilience of the concrete where other materials are embedded in it. For a fence post, these turning moments are probably limited to wind - negligable with a wire fence, until animals or other things contact the wire. Otherwise the worst case is surface area of the solid fence style cladding vs wind.
For an antenna support, the other feature is the weight of the concrete. It's common for motorway construction firms to put the tall lights or CCTV gear on a tall tube - maybe 6m/20ft and put the bottom in a fabric 1m3 bag, fill it with concrete and that weight of concrete , sitting on grass/soil/more concrete is heavy enough to prevent that turning moment from wind or even accidental contact tipping them over. 1m3 of concrete is about 2.5 tons in weight. Foam in a 1m3 hole has virtually no weight by comparision, has little structral strength, and the bubbles mean the contact surface can deform with pressure. Fair enough, a long tube burried in it has the pressure diluted by the surface area in contact, but the resistance to toppling from 2.5 tons of weight is considerably more than using the physical contact area of the foma vs soil.
I found a few fencepost videos, but the revealing one was this one below, where the contact area only resistance clearly was insufficient for a 4 x 4" piece of timber, let alone a tall metal structure. In this video the problems with the foam are quite easy to translate to a mast scenario. when you look at typical 60ft antenna masts - a 1m3 lump of concrete would be classed as small!
