If you haven't modified it, the Thorns VSBattleCore effect doesn't have a special battler motion associated with it, so it should use the guard motion. I tested myself in my test project (and found a bug associated with states, btw) and it does show the guard motion with the default battlers.
The picture of your golem battler is larger than a normal one, is it a dragonbones battler or something similar? The process for requesting a motion for one of those might use a different function than I use for a default battler. You might also need to add more wait time for the motion to have time to trigger? Does it work with any normal battlers for you?