Why do you think it's not a bug?
0021 had only the single line
#define RX_BUFFER_SIZE 128
0022 added the conditional above which always results in RX_BUFFER_SIZE being 128. Why add a conditional that is never used?
Many people with a 168 Arduino edited this file to reduce the buffer size. The new code would be nice if it reduced the buffer size for a 168 Arduino.