Your code golf example makes no sense.

    B_mean <- dt[, mean(b)]

    Dt[b<b_mean, by=.(a)]

Unlike the dplyr solution the dt solution is robust and we can independently test to make sure the mean of b makes sense.

The very easy to reason around concept of dt[rows, columns, groups] makes the code extremely clear.

Your translation example is absolutely bonkers because it’s trying to pigeonhole the simplicity of dt into the nonsense that is dplyr.