That US 6 month number excludes field training (typically 1 year) whereas the 2-3 year German number includes it (6 months I believe).
This largely stems from a difference in how academies work. In many countries, field training is required to graduate. In the US, field training is required after you graduate in order to get a permanent job. This skews the total training time numbers.
That said, American police are still undertrained by comparison.
Compared to other countries American cops aren't really trained at all.
In Germany the training period for a police officer is 2 to 3 years, in the US it's usually less then 6 months.