You do realise that the new champs group is not the same as the rework group, both of which have nothing to do with the post release balance team, right?
Why on earth would they hold off on releasing two more sources of income, especially after already saying that they will be released within a week?(people hate it when Riot says they are _looking _into doing something, and then decide against going through with it. Can you imagine their outrage when they not only said they were going to do something, but come 99% of the way just to hold off for an arbitrary reason? Riots(punpunpunpunpunpunpunpunpunpunpunpun) and lynch mobs would follow.)
Riot has hundreds of people working for them, and a sizeable balance team. 5-6 extra people will not make anything go faster(at a noticeable rate that is), and it would be stupid to hold off new champs and reworks(which generate income) to balance(which doesn't), when it works to do everything at once. Riot is a business, and they make decisions based on that.