"Accessing any address [on 6502] higher than the zero page (0x0 to 0xFF) causes a performance penalty."

I never thought of it that way, but that's true. However, he didn't mention the biggest issue with C on the 6502, i.e., the extremely constrained 256-byte hardware stack. To do anything practical requires some sort of software-maintained stack to have stack frames of any decent size or quantity (in parallel, or replacing the use of the hardware stack completely). "Downright hostile to C compilers," indeed.