That's utter BS. Aluminum and/or carbonfiber have less resistance than air, and electricity always follows the path of least resistance. If a lightning strike was well and truly harmless, you could never know to a certainty that you've been struck.
It tends to superheat whatever it hits and burn a hole where it goes in and another when it goes out. Lightning strikes are most common in the 'teens' (13,000-19,000 feet) and at 32°F, which is where turboprop "puddle jumpers" spend most of their flying lives, so they are most at risk. Spinning propellers/rotors collect a lot of static charge (remember the guy who got shocked by the helicopter in Hunt for Red October? That's fo realz!), which attracts lightning, so the prop is the most common point of entry.
There's always a chunk knocked out of the prop as the "entry wound." From there it passes through the engine and it exits from the trailing edge of the flaps or ailerons, burning an obvious hole in the skin as it exits.
THAT is how you know for sure you were hit.
And it also trashes the (turbine) engine, overheating the main bearings so they will anneal once they're allowed to cool. It probably will continue running until you can make an emergency landing, but once it shuts down the bearings become soft and even if it would re-start it's unsafe to fly.
If there's a bright flash outside the cockpit and one of the generators gets knocked offline, that's a pretty good indication you've been hit. Sometimes the generator will reset, sometimes not, but in any case you need to be looking for the nearest suitable place to land.
FWIW, statistically, every airliner in service is due for one lightning strike a year.
Yes very dangerous