There is tension on the forearm muscles at the bottom of a preacher curl from the stretch, and an isometric contraction as you keep your hands fixed while curling the bar. If you’re flexing your wrists during the curl you may even be doing a wrist curl inadvertently as well.

I can’t say whether putting elbows out is unsafe or not, but if it’s putting your biceps in a more advantageous position to lift more weight it’s probably not worth it. Remember that your muscles only care about tension, not the actual weight you’re using. There’s no reason to adjust your positioning to lift more weight - use the weight and position that maximizes the stretch and tension on the muscle you’re targeting for your set and rep targets. No reason to make it easier just to lift more weight if your goal is hypertrophy.