Communities are truly communities when they are open to others, when they remain vulnerable and humble; when the members are growing in love, in compassion and in humility. Communities cease to be such when members close in upon themselves with the certitude that they alone have wisdom and truth and expect everyone to be like them and learn from them. – Jean Vanier
Radical Discipleship 
Communities are communities because the people inside have something “in common”. Insofar the people inside are indeed “like each other”. This Bartimaeus thing here is also about having something in common.
This obviosly doesn’t imply that they alone have wisdom. But it implies that they don’t want to continuously explain or defend their point of view against outside people who differ in the basics. Also they can learn from outsiders only insofar as outsiders have found solutions to problems which the insiders share.
That’s simple common sense (and I wonder if Vanier has really not seen it).