In some way, it is; better minification does give you some additional margin. But it would then be a terrible code golf competition because 13 KiB here is the size after zip archiving so the actual limit is anywhere from 30--40 KiB for typical entries and would be very generous. The introduction of better compression tools may have lifted the upper limit a bit but not too much. (Disclaimer: I'm the author of one of such tools that got popular later.)

It should be noted that most assets are still too large in this limit, so almost everything should be made from a code and many stuffs have to be consciously chosen to be included. For this reason, I feel that the limit is rather a very interesting threshold that allows for ordinary developers without much size optimization experience to start with, but should be plenty enough for experienced people to make difference.