I don't like the article's use of the word "snares" - it's not as if it's a trap they're being sneaky about. There are signs everywhere, there's painting on the road, there's continual press coverage, and there are wide and well-signposted turning areas for private cars on either side of the bus gate. Anyone who gets "snared" by this is clearly driving while not paying attention, and deserves more than a fine IMHO.
Looking at it on Google maps, the road markings and signs are very clear.
Although I do see an issue with routing: Getting from Albert street to Selfridges by car takes twice as long as walking. But that is not the bus gate, but die to many roads being closed for HS2 construction.
Birmingham made a shortcut for bus users. I'm not sure the city council would be able to implement income based fines itself, so the flat fines are probably the best that they can really do.
You can judge for yourself if the signage is adequate from the council's photos, but I think the fact that I can see that the lane is painted green and says "BUS GATE" on it from the thumbnail image kind of gives it away.