Sorry but this is incorrect on both key points.

First, grout is not waterproof. It doesn't pass water easily but it's not waterproof.

Second, the reason you caulk changes of plane is precisely because caulk is flexible. The two planes will swell and shrink at different rates over time, causing grout to fail. Having said that, grouting a mitered tile assembly around a niche is quite common because the one plane is so small it won't move much.