Yes: BRIN indexes don't provide any ordering information. A btree index on created_at could be used to optimize this query, but without one of those, seqscanning the whole table is the only possibility.
Thanks Tom. So, this is a very general question, but would it be possible to develop that feature into BRIN, given what it stores? Even if it does not have ordering information, doesn't it know which blocks would contain the lowest values, so it could choose to do a "bitmap scan ordered sort" or something, depending on the number of rows sought? Or is the problem that it has no way of determining what value actually would be the "minimum" without the query specifying a particular date, such as less than "2013-04-01"?