The Force can be with groups and individuals who haven’t trained before. Luke and Leia had been with the Ewoks, and their thoughts might have been a catalyst for the Force to be in their presence. Also, the Endor fight concerned the Force on both sides, so there could have been a heavy Force presence in general.
The Force being catalized by Luke didn’t help Biggs, Porkins and the other pilots who got shot down over the Death Star. In Star Wars, we see the Rebels suffer heavy losses until one pilot, who is strong in the Force, manages to land one crucial shot. In Jedi, there are dozens of Ewoks, who are fighting for a long time and apparently have the force, as they have virtually no losses. Chewie stealing the Walker was only necessary to get the Imperials to open the bunker doors, not to turn the fight against the troops that were already outside.