> Doesn't C99 compiler produce the same machine code for ANSI C source code?

Only if a C99 compiler is available. As the post points out:

> The slowest of the “big compilers” to adopt C99 was the Microsoft Visual C++ compiler, which did not adopt it properly until 2015 and added more compliance in 2019. A large number of our users/developers are still stuck on older MSVC versions so not even all users of this compiler suite can build C99 programs even today, in late 2022.

(emphasis mine)