PSA: Thresh is a ranged champion
He can't execute with relic shield. He has to last-hit normally in order to proc it.
The auto-attack animation is weird, so getting it mixed up is understandable. Also makes sense thematically, since Wardens aren't supposed to execute on the fly.
Edit: I don't mean don't take it, it's a wonderful item on Thresh, just remember the execute passive doesn't work. That's all I'm saying.