"'when you use SRC'. So I used SRC to test the statement. It produced the exact same binary. So I conclude the statement is incorrect."

I have a feeling it's incomplete, rather than incorrect:

Inline assembler in Keil requires two steps:
1. specify the SRC directive on the command-line when compiling the 'C' source file in question;
2. place ASM/ENDASM pragmas around the inline assembly code within the 'C' source file.

I have a feeling that C51 may ignore the SRC directive if you haven't actually used any inline assembler within the 'C' source.

I'm afraid I can't test this theory just now.