I've done similar repairs, and also repaired other's repairs to strengthen them. I also agree this is a replacement piece of wood, and not a bad match, but should have been fitted at an angle to give the repair joint more surface area. The longer the angle, the more overlap the existing wood, and the repair will have. This will also make the repair less visible if the joint is angled and longer. A butted repair stands out much more!
If I think the area is weakened by a repair, I remove it and redo the joint the way I like. I also remove excess wood on the inside, and use thin fiberglass cloth, and Acraglass it in place on the inside where it wont show, to strengthen the wood, and the joint. I use Acraglass to install donor wood also, as it's stronger than the surrounding wood.