Let's ignore the disingenuous hair-splitting for a moment and contrast the two languages regarding extensibility.
Q: Can you extend the C language?
A: Yes, it is possible through custom preprocessors or modifying the toolchain. (C macros are shorthand, they don't extend the language.)
Q: Will what you get remain C?
A: No, it will be something else.
Q: Is extending C practical?
A: Not at all. It's hard and expensive work. It is done very rarely.
Q: Can you extend Lisp languages?
A: Yes, through macros.
Q: Will what you get remain Lisp?
A: Yes, the facilities to make macros are standardized and part of the language, the macros are part of the program.
Q: Is extending Lisp practical?
A: Yes, macros are easy to write and are common in Lisp programming.
Programming in Lisp is a completely different experience from programming in C. The language is no longer a rigid set of rules, a constraint, but something that you can freely form and modify as you see fit. It requires a different way of thinking.