I'm not an expert trader per se, but as far as I know, huge price moves are to be expected after long price stagnation, especially for low cap(at least compared to other assets) and heavily volatile assets such as bitcoin.
Take note though, when talking about the "calm before the storm", the "storm" they're referring to doesn't automatically mean a huge price move to the upward. It could easily be the opposite.
I don't think that such prediction is based on some concrete data or analysis, it's more like you said, some logical price movement after longer period without significant price changes
However, this is not something that we can take for granted, it might happen.or not. In fact in this case O think it's more wishful thinking. And this "price storm" could have negative effect so don't be surprised at the end.