Hard call that one, owner of the property said the horses had been chewing off the bark on trees in the paddock for the 10yrs he's been there, and 90% of the trees looked great except for the appauling damage!
Remember its only the phloem being compromised so disconnection between leaves and roots, this means the roots decline slowly, struggle to regenerate, struggle to maintain fungal associations....spiral of decline. That Burpengary property was a major minature horse stud mega $$s so they have CSIRO provide annual soil testing and fert regimes for the horses and indirectly the trees ofcourse, this is doubtless delaying the impact of the phloem damage/loss. I'd expect the trees to begin to swell above the girdling and over the next 5-10yrs (depending on the size of the tree) for the trees to show signs of decline dieback and failure....but I could be way out they might continue on for another 20-30yrs if they are constantly supplied with fert boosts.
There's a great PhD thesis on wild horse damage to trees in NSW national parks...too big to post deals with the issue very comprehensively.