Value of "process_name" is not "smtp" or "lmtp"?

In source code of src/smtp/smtp.c, there is a handling logic checking process name, and output log message "unexpected process name \"%s\" - specify \"smtp\" or \"lmtp\" ", so does this means that configuration option "process_name" should only be "smtp" or "lmtp". However, in the document of option "process_name" in http://www.postfix.org/postconf.5.html#process_name, it indicate that "process_name" is read-only. I wonder how could this happen if the process name will not be "smtp" or "lmtp"?