I am sure that to an extent the AMG engines are hand built. It probably comes down to the final assembly and inspection performed by one man, no engines can be completely built by machines, there are some tasks that have to be performed by humans. I am sure it is not just marketing, a big company like Mercedes would not falsely claim something like this.
The M156 really is an amazing engine, combining the attributes of a big displacement muscle car engine with that of an engine that likes to rev.
No matter if the "one man one engine" is just marketing or not it really don't change how the engine works. Let the poor guy be proud of his car just like you are of yours!
2011 E90 M3 ZCP - Individual Moonstone/Individual Amarone Extended/Individual Piano Black With Inlay:LINK!!!
1994 Euro E36 M3 Sedan - Daytona Violet/Mulberry:LINK!!!