The abstract and the rest of the paper don't really match imo. It's not really allocating more to some sequences, but just introducing ~dropout. Might be different sides to the same coin, but was still a weird read.
That said, it's a pretty simple change to make the approach work in the way you describe (allocating more to some sequences and not others) by changing the group across which the top-k works. In the paper we use the time (sequence) dimension, but one could also use the batch * time dimension, which would result in asymmetric allocation across sequences