I would believe they saw something they believe was Bigfoot. Doesn’t mean it was. Makes it more likely it was but they or others could still be mistaken. The level of encounter has a bearing on it. Say my brother and 3 girls were all skinny dipping during the day. Thus, they did not have their phones (or cloths for that matter.) Then, while in the lake all 4 report they watched for 10 minutes as Bigfoot creature eating crayfish on the shore a short distance away. After they got dressed - when bigfoot has reportedly left- they took pics and video of the tracks left by the bigfoot. Yes, that would come close to totally convincing me.
say the encounter was 20 boy scouts who - for 6 seconds- saw bigfoot on a path and then disappeared into the woods. This makes it less certain even if I trusted many the 20 people involved. The issue isn’t me trusting the people, it’s weighing the situations variables for evidence and plausibility. THEN, I weigh the credibility of the person in addition to that.
Some stories are not as plausible.