All LED christmas light strings I tested used steel wires on each bulb. You can confirm this with a magnet.
Steel rusts when exposed to moisture. If you use the LED strings outside, they will be exposed to moisture and they will rust within a few weeks. Making matters worse, most of the bulb bases poked all the way through the socket to where the string's wires enter leaving an easy path for water to intrude.
Solution: buy some dielectric tune up grease from your local auto parts store and fill each socket before inserting the bulb. Dielectric grease is typically used on the junction between spark plugs and their cables to prevent moisture from corroding the junction. The grease is non-conductive so it won't short the circuit. It will, however, block water intrusion and condensation.