I concede that you have some point, in that it is possible that (in the context of training) that the dogs consider what they are doing as play.
However, I've seen it "in the field" under uncontrolled circumstances as well.
There was a police officer I knew who was a K-9 trainer. At his home he had four "retired" K9 dogs in a fence in his back yard.
Those dogs, one in particular, didn't like me. The alpha male would engage in territorial-defense mode to an extreme degree, biting the fence and pulling at it until blood ran from his gums in an effort to get at me. I think there is no question that he was serious and that there was no "play" involved.
One day I went by there and he wasn't at home. The dogs were freaking out, and Alpha Male was biting the fence and pulling on it again...actually he was biting the GATE and pulling on it, and to my amazed horror IT CAME OPEN.
Out came four ex-K9 dogs, slavering for my blood. This was like my own personal Four Horsemen of the Apocalypse here.
I turned sideways and dropped into a fighting stance, determined to go to Valhalla with an honor guard.
The dogs came straight at me... and suddenly veered left and right, parting like the Red Sea, ran right past me and down the street, suddenly prancing and playing and looking like everything was super-cool.
I was left standing there, in my fighting stance, going "humina humina humina WHA' HAPPEN?? I thought there was supposed to be an apocalyptic battle? An Earth-shattering Kaboom?? Wassamatter, I ain't good enough for ya??" LOL.
Anyway, when I got a chance, I told the professional dog-trainer the story. She told me that it was because of how a dog's drives work. Yeah, those dogs wanted to rip me to shreds whenver they saw me... but they spent maybe 5 minutes a month thinking about killing me, and HOURS every DAY thinking about "boy I'd really like to pee on that mailbox, and run down the street and see those other dogs I can hear and smell half a mile away, etc".
When dogs are confined within a given space, like a fenced back yard, their territorial-defense drives are often "primary". The INSTANT they are no longer restrained to a specific space, territorial defense may very well take a back seat to other drives, depending on the circumstances. She said this is exactly what happened in my case; as soon as they were outside the fence they were no longer in their territory, so the defend-territory drive went secondary and other drives instantly came into play. They decided that eating me wasn't nearly as important as peeing on that mailbox and visiting that other dog down the road.
QED: Animal feelings and drives are different from humans. (and boy am I glad of it!)