I dare say that is true in some cases. For OEM and suchlike they commission their parts from Bosch, Mann, etc and put then in a nice box with a spangly hologram, same as all the other car manufacturers, but unlike some they just don't seem to charge over the odds for it. I was really surprised when I first realised that.
There are exceptions to that; this week I replaced a pollen filter that is only 4 months old. It was a Bosch item that was very flimsy, and blocked very quickly. I have replaced it with a MB one from the dealer. Its heavier, made differently, and hopefully will last a bit better.
I could offer more examples, but suffice it to say if I want service or maintenance items I just go straight to the dealer and save myself a load of time and agg for no extra cost