They were just as stupid as modern humans. I can point you to a hundred videos on the web of teenagers being incredibly stupid and doing things that result in their death or near-death all documented by their friends holding the stupid video camera. I can easily imagine a group of hominids looking at the Rock of Gibraltar from the African side and imagining they can easily reach it on a reed raft. Tens to hundreds of them may have drifted out through the straits before one or a group was successful. I can easily imagine one doing it.
Here's what it looks like from the African side. It looks tempting.
Been through there a number of times, the current can get real swift.
BTW, it didn't look like that during the Neanderthal period, anyway.