That's interesting. I had never heard that before. So the fake 2596's will look exactly the same as the 2576's, even the lettering?
I'd say except for the lettering, that will read 2596
See e.g.
here (the comments from Dan and Dave).
I took the attached image with a 1.75A load: 54kHz, about 1V of ripple and a hot chip (input 19V, output 5V).
Yellow track: output (AC coupled), cyan: switch output; spikes are due most probably to the jumble of wires.
No way to get it on 150kHz under any load/Vin/Vout combination.
And it didn't even come -directly- from eBay or Aliexpress, but from a well known Swedish distributor of small and home electronic: they took it back with no problems and "started an investigation" but the item is still on sale.
Re-labelling inferior chips is quite a common practice, plenty of examples on the net and on this forum.
Edit: added where to look in the linked article.