The leak comes via the generally accurate insider Tom Henderson, who has reported (through Try Hard Guides) what the full map for Warzone 2 will look like. Granted it's a pretty basic sketch, but you get an idea of the layout, how water will be incorporated, and likewise, where each major point of interest will be found, and what they will be named.
The main thing to take from this leak is that as a previous report from Henderson suggested (thanks, Exputer), the mention of a Quarry, an Airport, and a modern city does seem to align with the talk that the Modern Warfare 2 maps Quarry, Terminal, and High Rise will be incorporated into this new iteration of the battle royale map.
Otherwise, the report leaking the Warzone 2 map does state that there will be a new swimming mechanic, meaning players shouldn't fret at the sight of all the new water spread over the map.
As for whether this report holds any truth, we'll have to wait for Activision to officially reveal the Warzone sequel to find out.