On less than vertical terrain like this, you don't smash into the wall so much as skid down it. On easier routes, this can be more dangerous, since there can be more ledges to hit. But on this route, there are very few features on the wall at all, so the danger is small. Also, with pro so close, total fall distance would be minimal, further decreasing the odds of hitting something.

On steeper terrain, falls are even safer - assuming you are given a proper belay. With a good belay, you simply fall into empty space with nothing at all to hit. But with an inexperienced and nervous belayer, they might take in slack when you are falling, which is bad, since it turns the rope into a pendulum, resulting in you "spiking" the wall with significant force. Another danger is getting your foot tangled up in front of the lead line, causing the rope to flip you upside down when it comes taught, which has a significant chance of putting your head where you don't want it to be.

Adam Ondra has been climbing since he was a kid and has likely taken many thousands of lead falls over the years. His belayer is someone with lots of experience who he knows and trusts. If he thinks the helmet is unnecessary, I'm inclined to trust him to make his own judgements about safety.