There's another way around it. People that see themselves as "freethinkers" are also ultimately contrarians. Taking contrarianism as part of your identity makes people value unconventional ideas, but turn that around: It also means devaluing mainstream ideas. Since humanity is basically an optimization algorithm, being very contrarian means that, along with throwing away some bad assumptions, one also throws away a whole lot of very good defaults. So one might be right in a topic or two, but overall, a lot of bad takes are going to seep in and poison the intellectual well.
You don't have to adopt the ideas of every fringe or contrarian viewpoint you come across to be a freethinker; you simply have to be willing to consider and evaluate those views with the same level of rigor you give to mainstream views. Most people who do that will probably adopt a handful of fringe beliefs but, for the most part, retain a very large number of conventional beliefs too. Julia Galef is kind of an archetypal rationalist/free thinker and she has spoken about the merits of traditional ideas from within a rationalist framework.
