Pedantic cloud enjoyer here, the article should name this a pyrocumulus not pyrocumulonimbus - the nimbus part means 'rain-bearing' in meteorology, fire clouds don't rain.
Cumulonimbus has been separated from its etymological roots. It now refers to the physics that can produce deep convection, high clouds, and the formation of an anvil shape (along with lightning, etc). The convection that produces a cumulonimbus rain cloud is present in a pyrocumulonimbus cloud. The word isn't wrong.
Pyrocumulonimbus [1] appears to be the most extreme version of pyrocumulus [2] which, coincidentally, was observed halfway around the world in Washington State this week [3].
Incorrect. These fires basically create their own weather systems. All that smoke also contains a lot of water in the material that is burning. (That makes sense. They are trees, after all.) When the smoke reaches high enough in the atmosphere the smoke cools and water can precipitate out (rain) and also lightning potentially producing more fires.
During the press conference describing this the speaker claims they’re working with meteorologists from France Météo supporting the term at 6:30 https://www.dailymotion.com/video/xas3lw2
Technical details in the video link start around 3:34 for those interested
Excuse the French for the non Francophones but trying to be detailed and exacting here on why this isn’t bad journalism to use that term