A cult usually gives their members certain values. The members have those values the get into conflict with individuals with whom they have relationships with don't share those values. Then the cult member is encouraged to cut of those relationships because they hold back the cult member.

The social default is that you don't cut of relations with members of your family even if you don't draw value from those relationships. Groups that do encourage members to cut of their family connection then get seen by other family members as cults.

Members of a cult see other members of the cult has being high value relationships and relationships with outsiders as low value. That leads to group think and not being grounded in society as a whole.

The rationalist who cuts of relationships with everybody who he doesn't consider to be a rationalist falls under this case if you take the outside view.

with the implication that this is a generally sensible thing to do.

It seems to me that it is.

That ignores the point I made. In the outside view it's cultish behavior to cut certain relationships just because they provide you no value. With the inside view you always can find reasons.

I don't want to say that you should never cut relationships but having a reluctance to do so or recommend others to do is good.