That their original attack is placed into the future, when they very rapidly modified space (we know they can do it from 'Zero hour') and were stopped by the federation at Procyon 5 (which will take place as seen).
If you're suggesting that the sphere builders can later return, and try again? I highly doubt it. They know it wouldn't work. People in the regular universe know how to defeat them. They wouldn't dare try again. (Assuming they have even survived "Zero Hour", which is not clear.)