True, but the Russians claim to have done it during their experiments that produced the tame Silver Fox. That's why I mentioned it above. Most scientists think that if a dog-fox hybrid actually was produced, it would be sterile, like mules.
The Russians got very dog-like foxees by breeding for docility and tolerance of humans but I never heard they tried breeding them with dogs.
The tame silver fox was never interbred with other canine species, it was selective breeding within foxes. The important proof of that experiment was that wild canines could be selectively bred into tame canines and that foxes contain the same genetic variations that dogs and wolves have.
The russians bred tame foxes that ended up looking like dogs. I don’t remember them introducing dogs into the breeding program.