I mean.....he did it to Robin. Lol. Let her think she did something when he could have ended the fight the second he saw her (by just spawning invisible thorns all around her). Sommers is a weirdo sadist. He likes people to try and then watch them fail horribly, rather than just winning.
Not saying he is doing that here, of course (he was TRYING to do that earlier, when he thought he had Zoro trapped, it just didn't work out for him 😅). Though its genuinely bizarre he would run in this situation when a weaker Sommers was totally comfortable going against Gaban, once he got over his initial shock that Gaban was even there. Like, he clearly had no chance of winning there either and he still didn't RUN of all things.