I’ve bought several plugins from PB over the last few years and have found them to be very reliable.
The one Steinberg product I bought from them was Backbone. After paying through PB, I had to download, register and authorise the software through the Steinberg website, just as I would with any other Steinberg software product. So it ends up being no different except, as you point out, you get a free plugin from PB as part of the deal.
I can confirm. I have bought a Steinberg product at PB, too. Same procedure as usual. PB is an established UK based company so there’s nothing to worry about.
Enjoy the free plugin (that’s how I got Backbone btw )!