What I am reading from this is...
Mantle is, by an large, extremely similar to the APIs used in next-gen consoles, as first suspected by Anand and others. Microsoft and AMD, however, denied this later on, most probably because of political reasons, and possibly to their credit, due to custom copyrighted changes in each console; so technically the propriety APIs may not be exactly the same but VERY VERY similar nonetheless.
This is exactly why some developers are pushing for Mantle on PC. Because the difference between PS4 API is so miniscule that the associated costs (from switching PS4 API to Mantle) are tiny compared to the gains with Mantle over DirectX. If PS4 had an entirely different API, nobody would bother implementing Mantle, given AMD's market share, and financial hurdles.
Playstation 5 vs XBox Series Market Share Estimates
Regional Analysis (only MS and Sony Consoles)
Europe => XB1 : 23-24 % vs PS4 : 76-77%
N. America => XB1 : 49-52% vs PS4 : 48-51%
Global => XB1 : 32-34% vs PS4 : 66-68%
