Is something that is very rarely really checked in reviews, but the rule is across manufacturers for such zooms.
A user on the FM forum, owner of a Z 400/4.5 VR S + Z TC-1.4x and a Sony 200-600, did a simple test to measure the actual real focal lengths of the two lenses.
The reason he did run this test, because he noticed right away that the Nikon 400mm f4,5 with the Z 1.4x TC attached seemed to give him greater subject magnification at the resulting 560mm focal length than he was seeing with the Sony lens set to 600mm.
Here are his results
1) The Nikon 400mm f4.5 + 1.4x TC has an actual focal length of
582mm when focused at a distance of 5 meters.
2) The Sony 200-600mm lens, set at 600mm, has an actual focal length of
545mm when focused at 5 meters.
https://www.fredmiranda.com/forum/topic/1816165