"Smarter" and "capable" aren't the same thing. Especially if "more capable" is interpreted to be about practicalities: what we mean by "more capable" of doing X is that the population, given a chance is more likely to do X than politicians are. There are several cases where the population is more capable in this sense. For instance, the population is more capable of coming up with decisions that don't preferentially benefit politicians.

Furthermore, the median voter being smarter and the voters being cumulatively smarter aren't the same thing either. It may be that an average individual voter is stupider than an average individual politician, but when accumulating votes the errors cancel out in such a manner that the voters cumulatively come up with decisions that are as good as the decisions that a smarter person would make.