Partially right.
In the UK (opposite in Germany), when turning right and making a hand signal at the same time, only the left brake is available and as you say, this could be very fierce and send you over the bars. A rear brake with the left lever will slow and steady the rider. Therefore the safer option.
However, if riding fixed, with only a front brake, the tradition in the UK, is to have the lever on the left. When turning right and hand signalling, back pedaling will slow and steady the rider and the front brake available as well.