The B5 really does make a lot of sense, the M5 should fear it.
Alpina is an interesting company, and their role in the marketplace is always easy to explain. They are best described as a tuner, although they are technically classified as a manufacturer, and their cars wear Alpina badges exclusively. They work closely with BMW, whose cars are the basis for Alpinas, and you can buy and service your Alpina from a BMW dealer. But they aren’t quite the same as BMW’s M division, as they are far more focused on luxury.
Yet even with the focus on luxury, the Alpina B5 is still seen as something of a competitor to the M5, and indeed the performance stats are similar. Check one out, testing and looking very composed at the Nurburgring.