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The Alpina B3 does indeed sport a twin-turbo engine - the older 3.0-litre straight six that used to underpin the 335i until BMW decided it could do the job with a single turbo (which they call, somewhat mischievously, the Twinpower turbo). The trouble is BMW no longer makes that engine, so Alpina has got hold of the tooling for it and is now making it themselves. In tuning circles, this is called going the extra mile.
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I've been a bit skeptical of this claim since it was first published.
First of all he asserts that the older engine - the N54 is the only one he could be referring to based on his words - is no longer in production which we all know is flat out wrong. It is still in use in the Z4 35i and 35is.
Secondly, with Valvetronic, the B3 engine is no more an N54 than an S55 is, which is to say not an N54 at all really. If you remove everything bolted to the head, its basically an N55, no? I mean, I don't know whether they use a bone-stock N55 long-block or upgrade some of the internals such as timing assembly or cams or what have you, but why even talk about the N54 at all? In my mind they've taken an N55, removed the turbocharger and all supporting equipment, and replaced it with a twin-turbo setup.
If I recall there was some claim elsewhere about them using an N54 block casting, but it's not clear why that would even be necessary, and even less clear that it would effect the character of the engine anyway. And for that matter, it's not even clear what the difference is between an N54 and N55 block to begin with - or indeed if there is any difference at all. I guess I could easily answer that last part by looking it up in one of the parts databases.
In the end, if this engine has Valvetronic (and by the way, I don't know if Alpina even states anywhere that it does, do they? Not sure TBH), then its an N55 with TT setup much like the S55 although without the new closed deck block and other reengineering BMW has done to the rest of the long-block.
That all being said, there is every reason to believe that the S55 will be as responsive if not more so than the B3's TT N55. I just don't like the wonky over-romanticized embellishment about rescuing the N54 from the trash bin or whatever.