How does Seaborn calculate error bars when using estimators other than the arithmetic mean?
This information is in the documentation of Seaborn.
They show a bootstrap confidence interval, computed by resampling units (rows in the 2d array input form). By default, in seaborn version 0.8.1 it uses 95% of confidence interval, which is equivalent to a standard error. This value is a parameter that can be changed.