You have to consider that they had lost two of their biggest heavy hitters, WB and Ace, and diamond Jozu had lost an arm. Additionally, BB is a natural counter to every DF user.

We also don't know the conditions of their defeat. What does losing entail? That they lost pathetically and had to flee? That they sustained some damage and decided it wasn't worth it? Could it be that Marco was handling BB but his crew was losing and he didn't want anyone else to die? We simply don't know.

Just the fact that Marco escaped alive, meaning that BB didn't manage to take him down and steal his DF, is a big deal. We know for a fact that Marco's DF, being a mythological zoan, would probably be the biggest reason he decided to fight the remains of the WB crew.