Why BgWriterDelay is fixed?

In src/backend/postmaster/bgwriter.c , I can find the following source
code(PostgreSQL9.2):

/*
* GUC parameters
*/
int BgWriterDelay = 200;

...
rc = WaitLatch(&MyProc->procLatch,
WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
BgWriterDelay /* ms */ );
...
if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
{
/* Ask for notification at next buffer allocation */
StrategyNotifyBgWriter(&MyProc->procLatch);
/* Sleep ... */
rc = WaitLatch(&MyProc->procLatch,
WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
BgWriterDelay * HIBERNATE_FACTOR);
/* Reset the notification request in case we timed out */
StrategyNotifyBgWriter(NULL);
}

But I also found the following in postgresql.conf:
#bgwriter_delay = 200ms # 10-10000ms between rounds
It is now comment .
But according to the fixed code of BgWriterDelay = 200, even when I update
bgwriter_delay in postgresql.conf to a different value(eg 300ms),
how can it ovewrite the fixed 200ms in bgwriter.c ?

I also want to know, if it is not a bug, then what is the reason?