Steffensen is in decent shape. In his previous race he clocked 45.65 for 2nd on July 14 in Reims, on July 10 in Rome he clocked 45.28 and before that on June 9 in Thessaloniki 45.94-ish).
So without any knowledge of his circumstance, it may be that he felt some cramping or muscle twinge during warmup and decided to just go out there and try to protect himself by doing some kind of technical tempo effort and hope things might free-up and then he could apply some force. It’s just a guess.
But if you were the Paris meet director, you wouldn’t be thrilled with what you saw. I hope that doesn’t count against him for the future in Paris. On the visual evidence alone, that was just an extended stride-through so I’d be assuming that if it was still cold (and maybe wet) when he was warming up he may have decided discretion was the better part of valour.
Then again, if you were the meet director would you rather have an empty lane. I’m assuming Steffensen would have run it by someone in the Paris organisation; either that or he started to have problems in the actual race and was just trying to survive to fight another day. He seems to be moving forward pretty well with Berlin worlds in mind, so an injury now would be a disaster.