Some would say this is just a bug, like using length to get the length of an array.

While it is a bug (if only a documentation bug), I don't see the parallel or your point. The behaviour of length(@a) is documented, and the result is incorrect for all but arrays with one element. The behaviour of $field = $sth->fetchrow() isn't documented and it actually worked (if I remember correctly).

I think a case can be made for context-sensitivity in DBI::fetchrow_array

Really? What argument can be made for fetchrow_array that can't be made for other methods in general? In fact, given the name of the function, fetchrow_array, it seems to me to be the least likely of candidates.