First, I'm glad to hear this because competition is good and Nikon makes great products.
However, I have to comment a little on how carefully they worded their "claim"...
They have 57% share of....
Mirrorless
Full frame
Pro cameras
Selling for $5000 or more
At first glance, there are 10 cameras that meet that criteria.
However,
seven of them are Leica...
Five of those M-series that cost $9000 and are only bought by dentists (
). The two remaining Leica are SL2 models, part of the "L-system Alliance" with Sigma and Panasonic. Those SL2 cost $7000. Essentially, all these Leica are niche products that have very small market share and really don't compete with the Nikon Z9.
That leaves just two cameras, both more expensive than the Nikon Z9 ($5500). Those are the Sony A1 ($6500) and the Canon R3 ($6000).
You also might argue that the Canon R3 is targeted at a different market, a somewhat specialized "sports" camera with 24MP (versus 46MP in the Z9 and 50MP in the A1).
The Sony A1 was announced in January of last year and the initial surge of sales of it probably peaked months before either the Z9 or the R3 were announced in the Fall (just a few weeks apart).
There also have been numerous supply chain issues for all the manufacturers, making it difficult to purchase some of the cameras. You can see this in the backorders at various retailers. Just looking at B&H, of the 19 models of mirrorless cameras Sony currently offers, seven are out of stock... more than a third of their product line!. Nikon is doing a little better, with six out of eight of their mirrorless models in stock. Unfortunately though, one of the two that's currently backordered is the Z9. Canon is also doing a little better than Sony, with just two out of ten of their mirrorless now out of stock. However, that's a little misleading because four of the Canon mirrorless that are in stock are their older M-series, which are rumored to being discontinued soon in favor of the new R-series. Of the six Canon R-series, two are backordered.... And both are their newest models (R3 and R5C)!
Right now, of the three mirrorless, full frame, pro ($5000+) cameras, only the Sony A1 is in stock anywhere.
Good for Nikon that they're seeing success with the Z9. It seems a great camera and will be interesting to see if some of its tech trickles down to the more affordable models.