We have no evidence, this is assumption.
However dog ownership in Thailand does not follow traditional western patterns...which in themselves goes from lapdogs to 'Bullies" - dogs explicitly bred for violence.
Many communities see Soi dogs as an asset as they act as guard dogs to their territories.
Thailand has virtually no laws controlling ownership and use of animals and there is a lot of ignorance amongst owners.. a lot of it traditional.
One has to wonder if the poisoner was a foreigner who found him/ herself at odds with these dogs.
One problem with dogs is that once in numbers above 3, their behaviour changes into a pack mentality, and this tends to make them more aggressive.
So what needs to be found out is why he had so many dogs and what they were used for and how they behaved.
None of this is covered in the article