Adding 1.5 L of very oxidized beer to your batch certainly didn't improve the batch but it didn't ruin it either. The impact will depend upon the size of your batch of beer. As you noted, when making a yeast starter on a stir plate (or anytime you let the starter ferment to completion), it is best to decant most of the beer and pitch the remainder.
Some homebrewers make a smaller volume starter, around 1 L, and then pitch the whole thing while it is actively fermenting.